Problems with epoxy bubbling

Can anyone suggest what causes dime-sized bubbling when coating the bottom of a 10 year old cedar-strip canoe with West System epoxy? And maybe a fix for the problem? I removed the old epoxy, which had delaminated. This should have given me a clue that there was a problem.... SOMETHING is reacting and off-gassing, but I have not a clue what is going on.....

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RE: Problems with epoxy bubbling

You removed epoxy, but is there still glass? If bare wood, are you reglassing?


One of two possibilities come to mind- "outgassing" as you say, caused by applying a coating during increasing wood temperature such as in the morning- this usually creates smaller bubbles, but I've seen them dime sized and larger when there is glass being added; or contamination from common boat chemicals such as wax or silicone -did you remove the old epoxy with abrasives, and if so did you decontaminate it of wax/silicone first? It's easy to just pick up the sander and basically push the crud around until it hides in the deep scratches, where it is all the more difficult to remove.

RE: Problems with epoxy bubbling

Thanks for the response. I dimply removed the old stuff with a knife, and vvacuumed it. No solvents or abrasives or cloths used. I did apply the initial round in the morning, but the second time it was at noon and the problem still occurred.  It might have been the temperature change, but where does the

 gas come from in the first place?

RE: Problems with epoxy bubbling

Wood simply contains a fair amount of air, which changes volume with temperature and so migrates in and out of unsealed wood. But I don't think that's what is causing dime sized  bubbles- to be that big, the epoxy has to be well along its cure cycle which means it should also be highly adhered as well.


Which leaves contamination, which your description makes all he more likely. It's NEVER a bad idea to decontaminate a surface, especially an unknown surface, more especially a surface that has failed to adhere previously. Some cedars are fairly oily/waxy and so like teak should be de-oiled with a solvent wipe just before epoxy application.


You can remove your current failed coating with a heat gun and scraper, then thoroughly solvent wash, then sand to no finer than 120 grit, and reapply. Be sure to solvent wash just before epoxying, if there is any lag time in the process.


A strip built canoe without glass though? That's not typical...

RE: Problems with epoxy bubbling

From feeding your boat beans. :-)

Seriously, it's air that's trapped in the wood. Wood is made of open cells with tough cell walls, arranged in a linear pattern. That's what makes wood, especially one like cedar, so light yet strong.

When the wood is cut, sanded, flexed or just gets old, some of the cells are opened to air. As the air and the wood expand and contract through thermal cycles, air is expelled and drawn back in. Once the wood is sealed with epoxy or whatever, the process stops.

So the trick is to catch the wood after it's had all day to expel the air and just before it starts sucking the air  back in. If you coat it then, as it cools it will be sucking epoxy in and sealing itself.

Nemochad is right, though. Dime-sized dimples are pretty big for outgassing. Is there a possibility that there may be a pinhole into the boat behind the dimple? In that case, there'd be the entire interior of the boat supplying air. For that matter, water getting in behind the glass from inside the boat could have caused the original delamination. Do you see any discoloration in the area?




RE: Problems with epoxy bubbling

Thanks again for the replies.

Since this is a typical fiberglass on cedar strip canoe, there is really no place  for any quantity of air to hide (assuming air hides in the first place...).  If it is solvent or oil contamination, what do I use as  a solvent to prep the surface?

For some of the bubble areas there is a touch of black discolouration.  For others ther is nothing visible. 

Since the bubbles are usually near a join, I am wondering if the epoxy is reacting with the carpenters glue?  Or moisture in the joint?  (DOes water and epoxy produce gas?

RE: Problems with epoxy bubbling

Acetone used to be the go to product, but is made with less purity these days so it often contains stuff that's not good. You can use it as a less expensive first pass, then follow with a dewaxing product- most paint companies have a version. Also, don't use cloth rags, since it is very hard to know that they don't have contaminants from fabric softeners and such. Use plain white paper towels or paper rag substitutes, folding and turning often.


Now you're mentioning "fiberglass" on cedar though. If the bubbles are under the glass, then dime sized is possible from offgassing, since the glass holds the epoxy together while the epoxy is still soft enough not to adhere, effectively increasing the air trapping window. The seams in the strips hold plenty of air for this.  It sounds like the discoloration is the wood below, having been exposed to moisture due to the failed original coating. It sounds like the original coating failure is due to strip-seam movement, which let in moisture and discolored the wood, increased the moisture related movement (inadequately glued seams, not glassed both sides, excessive loads/impacts?), which caused further seam movement, which further stretched the glass/epoxy coating's ability to remain intact, and so on.


If you want the clear glass, varnish look you're stuck with grinding out the bubbles, reglassing the spots, and fairing it smooth. Aim for a sanded, tapered bevel of the current glass, mask a little ways beyond the sanded area, add a piece of wet out glass over the full sanded area, then block/wetsand it smooth after it cures. If you removed "a little extra" wood while sanding (and chasing out the discoloration), you'll need to add a couple passes of epoxy to build up the low spot.

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