Polyester Resin - Follow up to the Epoxy Strength Test

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Last week I performed a mechanical bond stress test on an epoxy joint. (See that post here) The personal goal and reason I ran these tests are to determine if I must grind the entire surface of my boat deck down to raw fiberglass.  A quick recap may be in order.  We removed the teak decks (7000 or so screws, teak boards, the dense uneven bond of resin based product and seam caulk.) and repaired several places that had core rot.  The rotted spots between the fiberglass now have hardwood mahogany rather than fiberglass to match the construction of the original work done by the Ta-Shing shipyard in Taiwan.

While I am confident in my repair and filling of all of the screw holes with a syringe, I want to be sure that after doing all of this work, I don't have a water intrusion problem to the core again.  That coupled with the added benefit of applying some additional strength to compensate for the removal of teak boards is why we will put a couple of layers of fiberglass down on the whole deck.  Almost all of the feedback I have received indicates that the removal of teak will NOT reduce any strength in the boat, but I still think the additional layers certainly won't hurt.  If I was building a light weight racing boat, I might be worried about weight, but in the end, suspect that adding a couple of hundred pounds on a 58,000-pound boat just doesn't matter.

The test last week showed impressive results, but now I want to see if I can do this same thing with Polyester resin which is about 35% of the cost of the epoxy resin.

This weeks test is to simulate the adding of 2 layers of woven roving on top of my existing deck with two different types of surface preparation.    I used a section of the deck that I had cut out and removed when doing repairs (I knew they would come in handy for something).
The first part was ground all the way down to raw fiberglass, and the other section, much like in the last epoxy test was just sanded somewhat smooth with 36 grit but left some of the old gel coat and fairing material on the surface.

I picked up resin and catalyst from Ace Hardware because of it being fast and easy.  After doing some research saw that the brand they carry is by American Plastics Incorporated who makes resin for many different companies and private labels it.

After cutting my woven roving pieces to fit my test section, I mixed up the solution of resin and catalyst.  The manufacturer recommends a 1% catalyst mix so for 16oz of resin; I needed four ccs of the catalyst.  I am using MEKP catalyst for this test and mixed it for just over 1 minute.


The supplies we use for this job are listed below if you find this a convenient way to shop for them.    If you use these links, you get the same great Amazon pricing, but we get a small affiliate link credit.  Thank you.


After applying a thin coat of the resin mix to the surface, I pressed the first layer of woven roving down against the tacky surface.  I then poured a bit of the catalyzed resin onto the mat and used a 3" chip brush to dab it down into the mat until it was translucent.  It is at this moment in time I realize that I forgot my fin roller at the boat so used a small piece of PVC pipe to roll any air out from under it.  The pipe was not optimal, but I think that since it is just to check the bond, it is good enough for my test.

I then added another layer of mat and repeated this process to the still wet surface below.  I then added two layers of cross-strand mat as well.

I let this all sit and checked back on it about 26-28 hours later, and it was still wet.  I attributed this to the fact that the weather cooled off and it dipped from the mid-70s when I first did this to mid 50's overnight and into the second day.

Not having experience with this, I left it alone and let it sit another day.  When I returned now a full 50 hours later, it was still not hardened at all.  I was able to put on rubber gloves and just lift the mat off the surface by hand and fold it all up and throw it in a trash bag like a wet towel.

Based on the results, something went terribly wrong.  It may have been temperature or the amount of catalyst.  I had a hard time measuring the catalyst when I performed this test and opted to use a syringe that was designed for insulin since it had the CC measurements on the side of it.  I removed the needle and just used the plastic syringe.  Maybe that was the problem.


Either way, the test needs to be performed again.  This particular one was a fail.  I am reaching out to the manufacturer as well to see if I possibly did something wrong.

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How to prepare the surface for new Epoxy (Definitive test results)

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Midway through my deck project, I am being faced with a real concern with my decision.  I had planned on replacing the rotted core sections of the deck with a mahogany hardwood core and then covering it with epoxy resin and woven roving to build the thickness back up to what the original was.  What got me thinking was the cost of the epoxy at first.  I am not one to save pennies and risk project results, but the numbers were significantly different enough that it was worth inquiring.  

To put the cost difference in perspective, I am using standard retail pricing here.  To use 1 gallon of Epoxy Resin with the required hardener, it is about $175 for the resin and hardener.  To do this in Polyester Resin, it is about $45 for a gallon and the associated hardener (catalyst).  This not be a fair comparison as the epoxy and hardener combined will provide about 1.5 gallons of material, and the polyester will give you about 1.1 gallons due to the much smaller amount of catalyst required in polyester resin.  But this is still a huge saving.   Just for material, the polyester would cost only 35% of the Epoxy with epoxy running about $116 per gallon and polyester running about $41 per gallon of finished product.

 So I went out to a few respected boat repair forums and posed the question, Polyester or Epoxy and provided background on the work that I was going to be performing.  I want first to state that people are always generous and amazing with sharing their experience and advice.  That said, this turned into a bit of an argument amongst the 'forum experts' (I referred to them like this as they are respected contributors within the forum, but I don't have personal experience with any that provided guidance there) as to which was the better solution.  

The tools we use for this job are listed below if you find this a convenient way to shop for them.    If you use these links, you get the same great Amazon pricing, but we get a small affiliate link credit.  Thank you.


The challenge was that the answers were many times epoxy and many times polyester, and a few times even suggested vinyl-ester, but all of them were adamant their method was the correct one and all cited success examples with their methods.  In the end, I came to the conclusion that either would likely be more than adequate for my job as long as I prepared the surface correctly and followed the manufacturer's instructions for application, temperature, and preparation.  
At this point, I bought into the fact that epoxy would have a better mechanical bond between an old and new surface than polyester but wasn't sure if the "better" difference was required, but I started to lean toward going with Epoxy at this point.  

The details of this job are essentially removing all the teak from a teak boat deck, replacing the deck core where it was delaminated and then adding some layers over the entire deck to thicken the whole thing up and ensure none of the 7000 screw holes were still water ingress points.  So the question  I still had was surface preparation.  Did I have to grind all of the existing deck all the way down to raw fiberglass or could I add these layers of fiberglass over the top of a relatively smooth deck and then fair over the upper part of it. 

It was time for a test of my own to show how well the bond would be between raw fiberglass and that rougher deck surface that still had some gel coat and maybe paint and maybe some epoxy bond that was holding the teak down.

I used a piece of the top layer of glass that I had cut out of the boat as my test material.  I smoothed a 5" wide section of the test material and then next to that I ground a 5" wide section all the way down to the fiberglass.

I also cut two strips of 1/2" plywood that I would adhere into these two sections.
I then mixed up my epoxy and thickened it.  I have been using stuff I just bought off the shelf at West Marine for the start of my project, so I continued using West Resin 105, Extra Slow Hardener 209 and a 405 thickening fillet material.
I mixed it up to a peanut butter consistency and put a thin layer of thickened material on the back of each piece of wood and the two spots on the test material.

I smushed (yep that is a word today) them down onto the surface and set a tool box on top of them to apply pressure while it cured overnight.

It was now time for the test.  My goal was to pry each piece of plywood off the surface and collect 2 data points. The first one was how easily the material came off and the second one was where the separation happened.   

I used a pry bar and started with the board attached to the epoxy, gel coat mixed surface.  This piece was tough to get off the material.  I struggled to get it to come loose even with the pry bar, and when it did break off, it split the layers of plywood leaving much of the wood still attached to the test material.  I used a metal wedge and continued to break more of this off, and where it did come off, it had a bit of white stuff on it, indicating that the epoxy used to glue it down adhered to the existing epoxy and gel coat very well and pulled some of that loose.

I then repeated this on the side that was down directly on the raw fiberglass and was surprised that it came off with only about a third of the effort, so much easier to pull off than the side with an existing layer of old material under it.  The place it failed was interesting.  It didn't fail at the epoxy joint that I made.  That stayed intact and what pulled up was a layer of the raw glass.  So while it broke easier, the bond that it made was stronger than that of the side with leftover material on it.  

Based on the results, I have come to the conclusion that I don't have to take the time and effort to grind the entire deck down to raw fiberglass.  The test results showed that it had a more even bond, it was weaker than applying the new surface right on top of the existing material as long as that material had been prepared properly.

This test was a great experiment for me to do.  I was able to use my particular material from the deck, with the same material that I am going to be adhering to.  I know that this test is accurate for the work I am about to perform.  

There is one more test that I think I want to do before I add glass to the entire deck. That is going to be to compare these results to what they would be with a polyester resin based product as well.  I think when I do that next test, I am also going to more closely mirror the goals of my project by replacing the plywood in this test with layers of woven roving which is what the ultimate deck will have applied over it.

If you are embarking on a similar project and have questions about the test details and or my results, feel free to reach out to me.  The best ways to contact me are via email or one of the social links listed on this page or even comment on the video on youtube. 

If you are receiving this blog via email or on a device that won't play the embedded video, click this link to the video.  The link is  https://youtu.be/V1bXiKPYDKI



     
          

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Adding Fiberglass to the Deck where the Core has been replaced

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After all the work on the deck with replacing the core, it was time to start to get the glass laid down over the core.   While this post is a week or so after the last one related to replacing the core and smoothing a layer of thickened epoxy over the top of it, I did do this about 48 hours after that work and just took me a bit to get to writing the post.   LOL, I was busy taking advantage of all the time I could to get work done on deck and remove epoxy resin from myself.

Side note, it doesn't seem to matter how much I protect myself with long sleeve shirts, thick gloves, etc., It never seems to fail that I end up with resin somewhere around my wrists.  I usually don't notice it while I am cleaning up, I see it the next day when there are about ten arm hairs permanently bonded together with only one way for resolution.  And that is RRRIIPPPP, AAAAGGGGGHHHHH,   OOUUUCCHHH, That smarts!
Then I repeat this process next time I do work with epoxy.

As I looked at my project ahead, it was to ensure that I got at least one layer, preferably two layers of fiberglass mat down over the deck core.  This plan meant that I had to cut the glass ahead of time to the correct size and shape and prepare all of my tools and utensils so that I was ready when the epoxy clock started ticking.

I had bought some fiberglass mat (1.8 oz cross-strand mat) from an old naval shipyard supply.  They had 30,000 linear yards of it, and I bought 20 or so yards that were about 40" wide.  Fiberglass rating goes by its weight.  So in this case, 1.8 oz means that 1 square yard of material weights 1.8 oz.  You can estimate that it takes about the same weight in resin to thoroughly wet it out meaning every square yard I cover will weight approximately 3.6 ounces.

I rolled this out on the deck and cut several sheets of it.  The first one I cut to fit inside the opening in the outer deck surface.  Essentially this layer was just going to cover the new core.
The next layer was slightly larger, and each subsequent layer would be wider to integrate to the ground edge of the adjacent deck as we added layers of cloth and thickened the new surface.

Every time I do any epoxy work where there was a gap of time of more than an hour or so since the last wet layer, I start by cleaning up the surface.  I very liberally use acetone and douse a cotton rag with it (old T-shirts work great) and then wipe the entire surface ensuring that the cloth is still wet when I do.  I then use a clean cloth and wipe it once more.

I started by wetting out the entire surface using a small chip brush.  I  may have been better served using a roller, but not being sure how fast I would have to work, I was fine just using this process.  After brushing the entire surface with a layer of mixed epoxy, it was time to lay the cloth down into place on top of this.  It was windy this day, so I had to put a few small weights down on the glass until I was able to get it pressed into the wet epoxy and wet out the upper surface.

After lining the cross-strand mat in the location I wanted, I started by pouring some epoxy in lines on top of the mat in a small section.  I started at one one and worked my way down to the other.  At first, my goal is just to get the surface thoroughly wetted out and then I do some more accurate work of ensuring every part of the surface is saturated and that there are no air bubbles between the mat and the surface below it.

I use a criss-cross pattern with my brush using more of a dabbing motion than a brushing motion.  It doesn't go on like paint, and your goal is to permeate the mat with epoxy so don't worry about trying to get a "smooth" finish like you would when using paint on a wall.

I then used a "fin roller" to remove any air bubbles from below the surface and ensured a good bond to the substrate below the mat.  When using a fin roller, you are not attempting to race or go fast. Doing so can start to lift the mat from the surface in back of the roller if you do.  Use the fin roller in slow strokes, and it is best to go from the center of your work toward the outer edge to avoid lifting the edges.

I then used small pieces of cross-strand mat ripped (not cut) along the edge to start to bridge the gap between the core and new mat and the edge of the existing deck.  I did this in 2 layers.  The first one was only 2 inches wide or so, and the second layer was about 6 inches wide.  Now that I have that gap filled, I will start to build thickness on the entire surface.

I am doing this with a 28 oz (shoot it maybe 24 oz, I don't recall now) woven roving mat.  This material is a cross thatch pattern that is much thicker.  I really like the way this stuff goes down as well.
I started in one place only to get a feel for how the woven roving will lay down.  Now that I know, I will get a larger quantity of this and cut large sheets out the size of the opening.

As an FYI, West Marine sells woven roving.  It ran me just under $50 for a sheet that was not as big as this hole in the deck.  There are many places online to buy fiberglass mat of this weight that ranges from $6-$10 per yard at 50 inches wide.  It pays to shop around, just be sure the mat you get is compatible with your resin (Epoxy, Polyester or Vinylester).

If you are receiving this blog via email or on a device that won't play the embedded video, click this link to the video.  The link is  https://youtu.be/WGeqmn9Wy2Q



     
          

The tools we use for this job are listed below if you find this a convenient way to shop for them.    If you use these links, you get the same great Amazon pricing, but we get a small affiliate link credit.  Thank you.


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"Run she may, but Shine she Must" - Deb starts to pretty up DreamChaser

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This week was another good week of progress.  For those that have been following some of our earlier blog posts on the deck core replacement, you know that we patched a section of the deck core about 2 feet by 5 feet.  We have decided to add an additional new skin of epoxy fiberglass to the entire deck after we repair any soft spots.  

So this week it was back to more tap testing.  I found some hollow sounding “thud” sounds indicated delimitation at best, core rot at worst.  I had a friend of ours that does boat repair, new construction and reconstruction take a look at the deck and provide his opinion as well.  He suggested that we make a few more inspection holes in the deck to determine if the core is rotted or just delaminated.  So I did.  The good and bad news is that the core was damp, but not rotted.   I am not sure what to make of this, if this was just humidity forming in the delaminated sections or if this was from the removed screw holes where I may not have patched it 100% yet as I filled those holes.  

Either way, it indicated another section of the deck that needed further exploration.  Deb was the star of the show this week while we made progress on the deck core.  I cut the upper skin, and she removed the skin and removed all of the core pieces.  The good news is that while it was damp, it was completed dry within an hour indicating it was not soaked even after the rains prior.  

Deb removed the core, and we ended up making the hole in the deck larger to correct a bit more delimitation we saw.  We were able to re-use most of the core material as it was still VERY solid and not damaged.  We sanded the boards rough and cleaned them in acetone to be sure that they would allow for high adherence of the epoxy resin we would be using to create the mechanical bond between the lower skin of deck glass and the bottom of the core.

Deb did a dry fit of the pieces of wood and marked each one so that we knew where they would go back into the deck once the resin and harder was mixed.  She also marked any boards we had to cut, and we took them over to our workshop to cut there.  

The day before this activity, I was grinding the deck.  Not just a little bit, but grinding like crazy.  I used a 4 1/2” angle grinder with a carbide cup blade (36 grit) and ground the entire length of the deck from bow to coach house on the starboard side of the boat.  I removed all of the stanchions and stanchion bases as well as the inner jib main sheet deck connection so that everything could be prepped correctly for these new layers of glass that will be going down on deck.  This work was loud and knowing that I was going to be doing a lot of grinding, I had my dust collection system that I have talked about in a previous post (Click here for POST) and also a shop vac for regular cleanup.  
I wanted to be sure that we were not getting fiberglass in the water or in the air to get on our neighbor's boats.  Sadly I did grind for about 6 hours.  It was brutal on the knees and back, but we got it all done.  Someone did complain about the noise to the marina just as we were finishing.  I do feel bad about that, but hoping that by focusing all of the work in one day, we were able to get the noise behind us.  In hindsight, I may need to take off a day during the week if I was going to do this rather than on a weekend when more people are at the marina.  

As I was grinding, and cleaning up my dust every foot or 2, Deb continued her work by keeping the area of deck she is repairing clean as well.   She got done with her work before I completed grinding, so she started to polish the Dorade vents.  We have these massive 30lb solid bronze dorade vents for air flow.  The finish is a horrible green/gray/brown mess of 40 years of exposure to salt air.  Deb polished this thing, and it started to look shiny and beautiful.  The shine was all the motivation she needed.  Deb picked up some bright red oil based paint at the store and will be painting the inside of the dorade bright red.  That wouldn’t look great on an old dorade box so those were both plucked off the deck and she sanded these down to bare wood.  I had re-done one of the dorade boxes last year, and the finish I put on it just did not hold at all.  It goes to show that proper preparation is key to a long and lasting finish. 


The tools we use for this job are listed below if you find this a convenient way to shop for them.    If you use these links, you get the same great Amazon pricing, but we get a small affiliate link credit.  Thank you.

She sanded them all the way down to wood and put a reddish tinted stain on the teak boxes so that it comes closer to the red color that will cover the inside of the dorade cowls.  We still have more polishing to do on the bronze, and sadly if we don’t do this a few times a year, it is going to turn green and patina again, but for now she looks great.

To quote our friend Thomas (who I know got this quote from somewhere else).  “Run she may, but Shine She Must.”  
Sadly our boat has been 180 degrees to that saying in that she runs good, but there is nothing that shines on her until Deb polished those large bronze dorade cowl vents.


More to come soon, including a video of laying fiberglass to build up thickness over repaired sections of the deck and in preparation for several layers of reinforcement over the entire boat deck.  We will have a stronger than original boat deck when finished.  It will be leak free AND much easier maintenance wise than the teak.

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Installing new Deck Core

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It is never an easy decision to embark on this kind of work as a DIY project or hiring it out.  The sheer economics of this one made the decision easier for us, but over the years we have become more comfortable with doing our work ourselves.  We have found that most of the time the work we do ourselves is as good if not better quality over much of what we have hired out.  That is not to say that there are not great marine service companies out there, it is just that we have had varied results when we have used them.
Just like people vary in character and quality, so do the organizations that exist in this space or any other for that matter.

This thought about vendors and money was the mindset that we had while going into this project.  I started by removing the loose or rotted board that were inside the deck.  The entire core was filled with 3 inches by 9-12 inch long planks.  I suspect they used these because they were scrap wood from something else or because putting them in the deck in small pieces made it easier to get the "curve" of the hull factored into the deck.

Some of the deck pieces came out very easy, and you could tell they were delaminated from the top and bottom skin and were not even bonded together in some places.  This hole I cut in the deck exposed about 35 or so of these boards and of those about 8-10 of so came out of the deck with no effort.  I was able to use a screwdriver to get under the edge and lift them up.  The rest took much more effort to separate from the lower skin showing me that as I moved from the original water ingress point, they delamination was not as bad and was holding the bond together in those places.

I planned to do this job in sections because I am not in a boat yard where everything can be covered.  I have an awning and have jury rigged up some water protection but it is not permanent, and I can't reliably assume no rain will get to the deck for an extended job.  I have decided to do this section by section.  As I was standing on the edge of the cut I made, I could feel and see that the lowest portion of the deck just forward of the coach house was also very severely delaminated and wet.
I extended the cut another 18" or so back.  This additional cut doesn't get all of the bad core out, but when I do the next section along the side of the coach house, I will be able just to cut a bit of the part I just did up and join the two sections.  It is not the best way to do it, but it is the best solution for our current situation, time and location.

Some people have asked how I make sure that the work I am doing does not make so much dust that it goes on my neighbor's boats.  With a palm sander, it is not too bad, but if using the belt sander or, heaven forbid, the grinder, it makes a lot of dust.  Some of my tools have a dust port installed on them, and I have a Fein portable dust collector.   The best way to describe it is like a shop vac on steroids.  It has several suction levels and you can either use it like a vacuum of you can set it to an automatic mode.
In this way, you plug your tool into the Fein, and when you turn on the hand tool, the vacuum automatically comes on, and when you shut down the tool, the vacuum goes off 5 seconds later.  It works very well.

I then swept up all of the wood, sawdust, fiberglass dust and what not and got ready to prepare the lower skin for adhesion of the new core.  For the new core to be bond mechanically to the surface, I needed it to be rough and clean.
It was already rough, but there were brown spots that resembled the board shapes, and I wasn't sure if this was just discoloration from the wood being in contact with it the last 40 years or if this was a thin layer of the wood fibers that were still sticking to the lower skin of glass.  I didn't want to chance it so decided to sand that off.
I started with 150 grit in a palm sander, and that was way TOO SLOW.  I quickly moved to a 36 grit belt sander which made the job take just 15 minutes or so.

I decided that since the boat had hardwood core, I had to go back with something as good.  I contemplated plywood, but it felt like a 'lesser' material than hardwood, so I opted to put mahogany planks inside the deck.  As a someone who has had a hobby woodworking, it was a painful decision to take a beautiful wood like this and bury it in epoxy and fiberglass, but I had to swallow my thoughts and move forward once my mind was made up.

I cleaned up my mess again, and we dry fit all the new boards into the opening.  The mahogany I bought was slightly wider planks at 5 inches rather than three, so we had to do a bit of ripping on some of them to get a good fit into the opening.
We numbered all of the boards so that when the clock was ticking on the mixed epoxy and we were laying them back in, we would remember the order they went in and their orientation.

I then painted the lower skin with a coat of epoxy resin and harder well mixed.  I used West Epoxy systems 105 resin and 209 extra slow harder.  I opted for the extra slow to give myself some more time if needed given that I am not an experienced user of these chemicals, nor have I done a deck re-core previously so wanted to have plenty of time before the epoxy started to kick on me.

Hindsight lesson learned:  The coat of epoxy on the inner skin was not needed.  I should have painted the bottom side of each of the mahogany boards instead as an added security.  This is a theory but if I had painted the bottom of the boards, it would create a barrier on the wood so that the next layer of thickened epoxy would have a solid surface to cure to rather than potentially having the wood absorb some of the mixture into the grains reducing the amount in the joint.  Again, this is just a theory as I don't think even a balsa wood would be able to absorb so much of it that it would weaken the bond or reduce the material in the bond enough to make a difference.  But the short answer is this step for me was not necessary, and I won't do it moving forward on any other spots we repair.

The tools we use for this job are listed below if you find this a convenient way to shop for them.    If you use these links, you get the same great Amazon pricing, but we get a small affiliate link credit.  Thank you.


I then thickened a batch of epoxy and used a small plastic scraper with notches in it to spread the material and lay each board into it.  This process reminded me a lot of laying tile in a home and the way the mastic is spread out.  The notches in the scraper give it a nice even coat and space to expand when you push the wood down into the material.

Deb and I broke this up into a team sport, but if you were doing this on your own, there was plenty of time for one person to do it.  I would mix the epoxy, then thicken and spread the next section and Deb would lay the boards into it and apply the pressure to them.
We did this until all were down and we let the material start to harden a bit over the next hour or 2.

We then used more thickened epoxy and spread it between the gaps between the boards.  Doing so gave us one solid surface out of all of these small pieces of mahogany.   We let that cure for 36 full days (it needed about 24 hours but the time of day we did the work did not allow us enough time start to lay the glass on top.

Once it the epoxy was cured, I did set some cross-strand mat, but we will cover that in the next blog post as we only put one test layer down (and in days after that found a mistake that I had to fix).  More to come next week.

If you are receiving this blog via email or on a device that won't play the embedded video, click this link to the video.  The link is  https://youtu.be/5PBNEMJmgpQ



     
          

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Testing for Delamination or Core rot on your boat.

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To a sailor or boater more pause than cutting holes in their boat.  It is one thing to drill a hole through the vessel for a new through hull fitting while sitting on the hard.  It is a whole other category when you willingly cut a hole in it with a saw.

So this introduction is where we find ourselves.  As most of you know, we have been pulling up teak decks on the boat slowly.  We are doing this because we are incessantly tired of chasing leaks.  Each heavy rain brings a new drip and in some cases a something that resembles more of a dribble than a drip.  The consolidation of all of these leaks can fill several gallons of water across several catchment devices.  Getting a real heavy rain, put that one little trash can next to the mast, and the other under the dorade.  Another one takes its place along the edge of the forward dog house and then a variety of them along the cabinetry in the salon.  Oh the galley, forget about it.  The leak here, I am embarrassed to saw is from along the coach house wall on the port side. It is a steady enough leak that we concocted a crazy little catchment that directs the drips (dribble in this spots case) into a large Tupperware.    So, in a heavy rain, it is not uncommon to dump gallons of water down into the sink.

We are steadily taking steps to correct this.  We know we need to replace the port side coach house wall so figured we would focus on the deck as the first step to reducing the advancement of the problem.  What is the problem you may ask?  Well, I suspect It is the teak deck.  The Teak deck's boards are thin, most of the screw holes do not have bungs in them because the boards are too thin to have one put in.  When you look at the deck, you can see some spots that still have bungs in them but at least 1/2 of the screws are either exposed or had been replaced with a larger screw.

3" bronze flat blade wood screws in some places went through deck into beams
The Ta-Shing shipyard in Taiwan used bronze flat blade wood screws on almost everything.  These screws were the order of the day, and I am impressed that they are all bronze, but over time some of these have broken off at the head or slightly below the surface of the teak.  I can tell where the boards had repairs done on them because they have stainless steel Phillips screws in them.  The screws that used as replacements are about one and a half the diameter of the ones that were originally used and in most cases, the screws are not just where the bronze ones were missing.  They are also about 3” to either side of every row of screws.  So consider the primary issue here.  We have screws that are going through the surface of the teak, uncovered and then proceed down through the top skin of fiberglass down into the deck core.
A Properly maintained deck with well-sealed bungs to cover screws
Not only did the manufacturer do this with a large volume of screws to hold the teak deck down, but whoever repaired this, or attempted to, added almost the same number of screws to the original number of screws that Ta-Shing used.  So in my estimation, there are over 7000 holes in our deck.

Properly maintained, countersunk and bunged screws along with properly maintained teak would have avoided these problems for sure.  But neglect can wreak havoc on a boat, and these decks have suffered that fate over time.

We made the tough decision to remove the teak decks and go over them with fiberglass.  That decision is done and made at this point, and now it is a matter of finding the places where we have delimitation or core rot.

Sample of rotted Plywood, the layers have just come apart
Core Rot is easy enough to explain.  It is where the material that used as a core between the two layers of fiberglass exposed to moisture over time, and eventually it rots.  This exposure will make the wood spongy and soft and lose its ability to provide additional strength.  If left in its current state it can begin to impact the structural stability of the boat.  If you think about the deck of the boat, it is essentially the horizontal support holding the hull sides in place.  When soft or spongy, the deck may give a bit here and a bit there allowing the hull sides to flex.  I am certainly not shipwright but this is an obvious problem to me, and even just minor flexing will cause undo stress on all joints in a boat.
The lower image shows the top layer delaminated from
the wooden core
It can show up as small misalignments in cabinetry, door frames, and even all rigging through bolt positions as well.

Delimitation is a little harder for me to wrap my head around the actual impact.  I will start by saying I know it is not good, but I don’t quite understand if the impact is as harmful from a structure standpoint as the rot.  I presume it is and also must be corrected.  Delimitation is when the layers of fiberglass have separated from the core, and the surface is no longer a tightly bound single unit but rather three individual layers.

I have both and in most cases I am not sure which is the chicken and which is the egg.  I suspect the thing that came first for us was the ingress of water.  So once moisture has been able to get to the core, every time the weather drops below freezing that wood/water combo freezes and expands applying a bit of pressure to the bond between the fiberglass layers and the wood.  That coupled with the moisture, humidity and freezing temperatures over time wiggles a little gap between the glass and core essentially breaking the bond between the surfaces.  Think of plywood that was not glued together but rather just 3 layers stacked on top of each other.  That is what we end up with in some places on the deck.  The good news is that it is not everywhere and the easiest way to test for it is a tap test.

Performing a Tap test and listening for a hollow 'thud' sound
Take a small hammer and tap every 6-8 inches around the deck.  Assuming you have some solid spots, you can hear a solid tap, tap, tap with each blow of the hammer.  When you hit a spot that has delimitation or rot, the tap turns to a hollow thud sound.  I started with cutting a few exploratory holes with a 2” hole saw on the port side deck, and in a sudden case of denial, last summer felt that the surface below was in pretty good shape in the area that I tested.  I was pleasantly surprised but still in some disbelief that the core was hard wood and not plywood or balsa wood, but remained skeptical and in disbelief that could be the case.    I poured epoxy into those exploratory holes months ago and continued with the deck.

The starboard deck would not let my most hopeful side assume this was ok.  I did the tap test, or should I refer to it as the thud test.  A section that was about 3 feet wide and 5 feet long all had a thud sound.  It went from the edge of the coach house forward about 6 inches in from the toe rail.  The issue seems to go down the sidewalk on the starboard side of the coach house as well, but I am focusing on the deck forward of that for now and will be working my way aft.  I opted to not repair vast sections all at once due to time constraints and the challenge of working an area that large before epoxy were to kick or set but I am getting ahead of myself.

First cut with worn out saw.  Did not work great but the
saw was in bad shape.
I needed more than a 2” hole to inspect the deck.  I decided to start with a square about 16 inches each way.  I swept the deck and pulled out my old and cheap circular saw that had a 140 tooth plywood blade on it.  This blade wobbled a little as it cut into the fiberglass and it made me nervous.  I then realized that I was ready to get a new saw that could be relied upon when needed.  The old one had a bearing or something that was going in it, and the shaft had some wobble which is scary on many fronts.  I opted to get a cordless saw and one that would use the same batteries and charger that my cordless drill use.  I purchased the Milwaukee 6 1/2” circular saw and splurged a little and got a good 40 tooth Freud blade for veneer, finish work and plywood.  I set the blade guard and fence so that the blade would only cut about 3/8” of an inch deep to allow it to get through the top layer of fiberglass but not too deep into the core.
Using the new saw and pivoting it down into the deck through
the top layer of fiberglass
I laid the forward edge of the saw on the surface of the deck, held the blade guard open with my thumb while still holding the left-hand grip on the front of the saw.  I aligned the blade direction and with my right hand flipped the safety switch, pulled the trigger and slowly lowered the saw into the work using the front tip of the fence/guard as the pivot point.   Once the saw fence was flush on the deck, I could let go of the blade guard with my left thumb and slowly cut the deck.  I did this in all 4 directions of my square being sure to overlap the cuts slightly so that the cut was all the way through and the corners of the opening met.  This would prevent the bottom corner of fiberglass from cracking as the cutout piece is being removed.  I was blown away by how good this little portable saw cut.  I am looking forward to using this on many projects.  When it first started, it sounded like the blade was going slower than other saws I have used, but I think it is just a lot quieter.  It cut ten times better than the one I had previously.

The tools we use for this job are listed below if you find this a convenient way to shop for them.    If you use these links, you get the same great Amazon pricing, but we get a small affiliate link credit.  Thank you.

I then used a scraper and flat blade screwdriver and a mallet to tap the blade into the cut and pry the cut part up out of the opening.  The best thing that could happen to you is that it is hard to come up.  Sadly in my case, it came up pretty easily indicating that it did not adhere well to the core at all.  I found a few spots that were stuck well, and in the photos, you can see why.

It was good to see that the deck was cored with hardwood
The core on this deck is a hardwood.  The builder used pieces of hardwood that were about 3 1/2 inches wide and 9-12 inches long.  They laid them long ways bow to stern and by using small pieces like this they were able to allow small gaps between the wood that would fill with epoxy resin when being built.  This method of coring provided not only places where the fiberglass would stick to the wood, but where it would also stick to the epoxy bone structure that gets formed around each board holding the two layers of skin up.
These are the likely original culprits for the water leaks into the
deck core. These are where the gas lines went through the deck
What I found in my case was wet wood.  Some of the pieces were also soft and rotted, but not all of them so this is good.  I will replace them anyway and believe I have found the largest culprit for this rot.

The starboard side of the deck held a deck box that also doubled as the propane locker for the galley stove at some point in the past.  That meant that not only did the deck have large bolts that penetrated all the way through and bolted below.  It also meant there was a large hole that went through the deck to allow for the gas lines into the boat.  Over time these were ingress points for water, and the worst of my wet and rotted core is in the low spot just below the location of the deck box as well as all around these holes for bolts and gas line.

Next up - Removing the core and replacing it.  The question now is plywood or hardwood?
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