Boatbuilding Blog

September 16, 2007

Epoxy Bond Failure

Filed under: Building - Before Flipping — tomlarkin @ 10:33 am

When I put the 6 oz glass layer on the boat I followed the standard advice to fill the weave of the cloth the same day. The rule is to add a few more coats of resin on the same day, adding each coat when the previous one was tacky. This is supposed to create a great bond between the layers, without the need to sand or to wash off any amine blush. I did exactly that, rolling on two layers the same day I added the glass. The final coat was complete at about 9 PM that night – a long, difficult day. The boat looked great! The weave was filled. I had dreams of turning the boat over in Mid-July.

A few day later I washed the hull and sanded the whole thing with a longboard and a random-orbital sander. While I did that I noticed a few areas with a strange feathered-looking edge where I had sanded the top layer pretty thin. One day I took a putty knife to one of the areas. To my horror, the putty knife slid under the layer and peeled off a long strip of epoxy!

Putty knife scraping off epoxy and fairing compound

I was shocked, but hoped it was just in the one place. It wasn’t. Everywhere I tried, I could do the same. Some places adhered better than others, but most could be peeled off the same way. Some areas had two distinct layers. It took a lot of time to do even one square foot, and I occasionally gouged through the glass layer into the wood. The good news is the glass layer itself seems to be attached well.

This was the largest piece I scraped off I spent weeks scraping the hull, from mid-July to early September. It was incredibly depressing, moving backwards instead of turning the hull upright and starting the interior. By September first I was done, and the hull was stripped down to the glass, sanded, and ready to move forward again. 

I still don’t know what the problem was. When I was adding the fill coats I reused the mixing buckets and the foam roller. Maybe the partially-catalyzed resin contaminated the mix. Maybe I didn’t mix the epoxy enough (although the layers cured fine). Maybe the humidity changed and dampened the hull between coats. I have no idea. I don’t think it was a bad batch of epoxy. I tried some tests with the same 4-gallon container and didn’t see the same problem.

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