Interior fiberglass bubbles

I just finished my first boat, a Wood Duck 12. My first time out was great! No problems whatsoever.

However, a couple days later I went out again and I noticed that the fiberglass on the bottom interior has bubbled up in several places. The bubbles are oval shaped, about 1/2"x2" and very brittle. I also fear I have a small leak.

I did not fill the fiberglass weave completely with epoxy on the interior to aid traction and save weight.

I want to be out on the water more than ever now, but don't want to make the problem worse. Does anyone know what is happening and how I can fix it?

9 replies:

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RE: Interior fiberglass bubbles

More info: there were no bubbles my first time out. Something unknown caused them to form.


RE: Interior fiberglass bubbles

Can you send me some pictures of the affected areas? Can you tell if there is air or water within the blisters? I am also concerned about what is happening and hope that we can get to the bottom of the issue.

RE: Interior fiberglass bubbles

Thanks for your concern Joey. The bubbles are filled with air and seem to be in a straight line, which is perplexing.  Here are some pictures.


RE: Interior fiberglass bubbles

Looks like someone dripped some kind of liquid contaminant on the wood prior to glassing. Like oil or something else that caused adhesion problems.

Cheers, Pat

RE: Interior fiberglass bubbles

Pat may be on to something. Possibly surface contamination or just plain old poor adhesion. Since there is no water in them, I would not worry. Could be that the air was always trapped and caused a blister once warmed up by th sun.


The fix is simple, grind the blisters off and patch the area with cloth and epoxy. 

RE: Interior fiberglass bubbles


I already put a coat of varnish on. Can I epoxy over the varnish? 

RE: Interior fiberglass bubbles

If it wasn't something dripped on the plywood, perhaps there was some contaminant on the FG cloth... perhaps tape on a roll of FB? Just a guess.

re epoxy over varnish... I would sand it off and put epoxy on epoxy, sand when cured and then varnish over. However, fix the bubbles first. Cut the bubbles out of the cloth with a sharp knife, or carefully use a rotary grinding tool, and then apply epoxy and a small FG patch. But... if you are have already varnished the areas, perhaps wait until further work needs to be done.

RE: Interior fiberglass bubbles

Nice WD! I had a similar thing happen on the bottom of my hull, but before I was done building. These guys are all right on about the soultion-you must grind down the bubbles, and recoat with epoxy. It doesn't look too large or widespread that you'd compromise strength very much, but you can add a small piece of fiberglass cloth if necessary. It looks like you have a good, solid fillet built up. This is wat it looked like when Iwrinkle" mce_src="" alt="" border="" hspace="" vspace="" width="" height="" align="" /> sanded a bubble out:

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