Epoxy Shrinkage (blunder?)

Submitted byJimD777 onSat, 01/18/2020 - 21:47

I'm building a small stripper. I used 3/16" pawlonia strips for the hull, and WRC for the deck. As is my usual sequence, I stripped the hull, flipped it, stripped the deck, removed the deck, faired the hull interior, and fiberglassed the hull interior. I replaced forms 15, 12, 10 6, and 3 back in the hull at points I had pre-marked, before the epoxy was fully cured. I noticed that the forms fit well low in the hull, but there was a gap at the sheer, so I stretched tape across the hull to bring it in tight. One day later, the tapes strips were slack, and the forms fit snugly in the hull. I applied hot-melt glue  to hold them in place.

I put the form risers back on their respective forms, flipped the hull again, and fiberglassed the exterior. I noticed that the forms kept wanting to fall out (hot-melt glue has lousy holding power) . After the fiberglass coat, and one fill coat, there is now a whopping 1/2" gap at the sheerline. In the past, when I have had a misalignment problem, it was always of the hull being too narrow for the deck, and  I solved it with spreader sticks in the hull. I have no idea how to push the hull back together that far.

If you have solved a similar problem in the past suggestions would be greatly appreciated. If the problem isn't solvable, please give suggestions on how to avoid it in the future. Please DO NOT post replies telling me that I screwed up , I already know that. Thanks.


Sat, 01/18/2020 - 22:37

On one build, I glued (with thickened epoxy) short (1.5" ?) pieces of wood strip along the hull and deck edges.

Set the sticks at right angles to the sheer line.

Spaced the sticks alternately on hull and deck, like 'teeth'.

Use enough sticks to keep the hull and deck aligned. Wrap tape across the joint in the usual way to hold hull and deck tightly together.

Epoxy the glass tape on the inside of the seam. 

Grind off the sticks and tape/glass the outside of the seam.

As they say,: "It worked for me."

The epoxy will continue to harden for weeks, longer if your shop is cool, so do everything you can to get things aligned soon.

Allan Newhouse

Sun, 01/19/2020 - 01:47

The problem isn't epoxy shrinkage. It is a problem of the timber being affected by changes in humidity.

If you glass the outside and the humidity increases, the hull will flatten out. If the humidity decreases it will curl in

The most extreme example I have experienced was a hull that was supposed to be 51cm maximum beam. After glassing the outside one day, a huge increase in humidity overnight changed the beam to 60cm. I sat the hull in the sun all day and it curled in to 30cm.

I waited until the hull went back out to about 51cm and glassed the inside and it stayed close to that.

Allan Newhouse

Sun, 01/19/2020 - 01:50

I just noticed that you said that the interior was glassed first.  I hadn't expected that so some of what I said doesn't apply, but I still think the timber changes more than the epoxy.

Thanks, guys. This is only my second time using 3/16" strips-I usually make them thicker, about 7/32", or 1/4'.  I also had problems the first time I used 3/16" strips-maybe the thicker strips are less effected by the weather? The weather around here DEFINITELY has fluctuated wildly with temperature and humidity. Of course, now that both sides are coated, its probably  going to stay the was it is now. I'm going to try replacing most of the forms, pulling the hull back in with clamps across the gunwale, and warming it up.


Sun, 01/19/2020 - 13:36

My construction sequence is different - I glass the hull outside, then inside, then reinstall the forms and build the deck. I'm still quite diligent in keeping things aligned as much as possible, though. And the humidity here (PNW) is pretty much 50-60% year round.

When I reinstall the forms in the boat, I use a #6 screw just below the sheerline. If necessary, I'll add a small block at the keel line using a spot of hot melt glue, though I hate cleaning that stuff up later. The (wood) plugs in those sheerline holes would probably offend some folks who clear finish the wood, though.


Sun, 01/19/2020 - 20:08

I had it happen once. I put ratchet strap(s) around it and a small block of wood on each side at the shear under the strap. This will force it in. I did this when attaching the deck to the hull. I had the form before the cockpit and after the cockpit in there. I glassed the outside seam and just did the areas where the straps were later as small pieces. Once sanded, you could not find those seam tape joints. I learned to always store the hull with forms in it and straps around it at a few points in the middle. I only had it happen once and I think I was forcing some strip bends not only inward but upward which causes some extra tension. And many strips following that path starts to add up.

Thanks, Jay, that sounds like a plan with the ratchet straps. I'm going to try strapping the hull to several forms while I work on glassing the deck underside, etc. I think I should reinforce the deck underside to be stiff enough to resist being pulled out of shape by the hull.

I used a LOT of SMALL strips to strip the hull, and I noticed that they seemed to want to spring away from the forms as soon as I took the staples out. I must have done just what you described with the strip bends.

I've managed to 'persuade' worse than that, you shouldn't have any problem, you can use a LOT of force without damage.

Heat also helps, but never go above about 70 centigrade - 160 Fahrenheit, as the glass can start to separate from the wood. As Jay said, ratchet straps and blocks of wood along the sheerline. 

Let us know how it turned out


In the phot0, I have a strap with blocks of wood pulling the gunwale back to the form. I'm happy to report that once this is done, the hull is the "right shape". Now if I could get it to stay that way? I think I'm going to use eight strap & block arrangements. 

I think I am going to need to reinforce the arc of the deck (rear in particular) to prevent  it from being pulled out of shape. My decks usually come out fairly stiff when the underside is glassed, but I think that even if I get the halves joined o.k, there will be so much force trying to pull out, that the deck may distort. An extra layer of "fat" glass, like 10 oz?

John VanBuren

Tue, 01/21/2020 - 17:05

Between forms you could use some of the "strapping tape" that is reinforced with what appears to be fiberglass strands. It is very strong and does not stretch. Plus if you fold over a bit at the end , it is relatively easy to remove once the epocy is set.

I had some a while back that I believe was a 3M product that was about 2 inches wide.

It might help you keep the hull in a more perfect shape. 


I hope this is useful,

John Van Buren


Tue, 01/21/2020 - 22:27

Once you have the bulkheads in, they will stiffen things up.

If you 'guesstimate' the bulkhead shapes from the nearby molds, the boat will be the designed shape.

first of all, put straps on at each form (and if would be better to put as many forms back in as you can bear) to pull the hull in, and if you can leave it that way a couple of weeks, the epoxy has still not completely cured, so that should help somewhat. You could also make the opposite of spreader sticks out of plywood (Good in tension) if you prefer

Secondly, you are overthinking the deck, decks are much stiffer than hulls because they are shallower, I would just glass it as you usually do. If you really want to put an extra layer or a heaver layer of fiberglass on, you only need to do this on the underside of the deck, as that is the part that will be in tension.

Thank you guys. John, I always use that type of tape in my builds, going  through a couple of rolls each build :) taping it together

In the photo, I'm using that filament tape to hold the deck down-on this build, the hull was a little to wide, like 1/2", except at the bow, where a little bit of Good Old Brute Force was necessary.  I tack-glued with epoxy in between the tapes, then did an internal tape. Thanks  Justin, I'll just make sure the deck is stiff enough.


Thu, 01/23/2020 - 15:29

First, at some point release the strap and see how mush spring back if any you get. If you still get the hull springing out a bit, I would just put a few 1 x 1" boards inside across the boat and screw into the strut to hold it in position for the taping process. NOBODY will see the screw holes when done and filled. I know we all want immaculate craftsmanship. I can relate, but sometimes you need to do what needs to be done. Once that deck is seam glassed inside and out, the boat will be solid and firm. (as you know). Epoxy takes weeks or more to fully harden and if you caught this early enough,  the hull might set into the new position.  I made seven strippers and every one needs a bit of persuasion here and there at the end when fitting the halves together. It will work out fine, don't worry. And as someone said, the deck, since it's shallow will not expand out.

Thanks again, Jay. BTW, I think I know what happened-I used pawlonia for most of the hull. Its very light, low-density stuff, and I think it absorbed/desorbed moisture very quickly, and I was so intent on getting the job done when I had the time (you know how that goes :) that I didn't notice the expansion. I have most of the forms placed back in the hull with the ratchet straps pulling it in. I hope/think its going to take a set. If it doesn't, at least I now have a way to pull it in.



Fri, 01/24/2020 - 11:50

Jim:I'm sure you'll get this worked out just fine. For most of us, it's a case of 'been there, done that'.

To new builders: You can build a handy moisture 'meter' (aka hygrometer) by taking a strip of thin cross-grain wood (grain running across the narrow width) and adding glass and epoxy to only one face. The changes in curvature can be quite dramatic if your shop humidity changes.

For me, once I glass one side of a wood core hull or deck, I try to get the other side glassed very soon. Any delay just invites a change in the weather and a change of shape. And, I'm quite diligent about 'mating' the hull and deck back together (with masking tape, usually) when I leave the shop - even overnight.

This will pull in easily.  One key is to not overdue any one place.  Once you do what has been suggested above and you are putting the deck on use straps or surgical tube to pull the deck in.  Do it at both bulkheads first, make sure not to over tighten any straps that are not over a bulkhead.


After I "got over" the idea of putting holes through my hull, I put the sticks across the entire hull, as Jay described. The screws made it easy to adjust the width.  I glues it together with slightly thickened epoxy. There was a barrage of clamps, filament tape, ratchet straps, and shrink wrap to hold it together (the photos are too gross for publication) . After all that was removed, it held together perfectly, no distortion of anything. Thanks!


Thanks for letting us know you were successful, looking good!