Bubbles in epoxy?

�Doing the first sanding on my Wood Duck in preparation for fill coats. On the deck/hull edge I am running into what looks like a long bubble in the initial coat right on the edge. I sanded most of it lightly but still am continuing to reveal more little dots. Should I just carefully sand these? I don't see a way to send a photo.

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RE: Bubbles in epoxy?

This is first wet-out of okoume ply? No fiberglass yet?

You want to finish 'bright' or are you going to paint?

Porous substrates that are warming as epoxy is applied / curing will outgas, forming small bubbles. First wet-out's typically where this happens as you have all those little pores thru which any gasses in the wood itself can migrate out as they expand. Once that first coat's cured those pores are sealed off.

If no 'glass involved those bubbles are of little to no consequence. You can sand thru epoxy to open them then they get filled with more epoxy as you build layers.

You describe a "long bubble" which is descriptive of when 'glass is being applied / wet out and the 'glass fails to follow the surface at a change in direction, leaving a void underneath. This can be problematic as that void is a weak spot; sometimes they can be filled with epoxy using small syringes, sometimes they need to be opened up for filling then more 'glass added above affected layer. A lot depends on just how long/deep/wide they are, and of course how finicky you are about how any fixes will appear once your craft is complete.

FInishing 'bright' demands care and attention at each step, the better to avvoid things that will spoil the effect once you're done. Paint can hide a lot of cosmetic defects. The risk is in thinking what you're seeing isn't structural when in fact it may very well be. Hiding that under paint is to be avoided.

As for posting pictures here? You can't, at least directly.

Image files are too large to store on the server hosting this website & forum. So you need to find someplace to post/host your images, then look for a way to get the address where they can be accessed, then post that address here as a link.  Apple's iCloud, Photobucket, Dropbox, etc. are what you want to become familiar with for this.

RE: Bubbles in epoxy?

   I had some small air bubbles where I didn't round off the deck to hull joint enough near the bow and stern. I used an awl to break open the bubbles then just filled them with subsequent fill coats. 

RE: Bubbles in epoxy?

Here's a link to a photo.  http://i66.tinypic.com/t6dxmd.jpg

The boat will be bright all over. There had been one layer of fiberglass with one coat of resin. I think I didn't sand fully thru the glass, as the main bubble just fell off in a strip a couple of inches long. The remaining bubbles are along the top side of the area. This was built in the October Wood Duck class at CLC in 2018, so I did have good instruction.

RE: Bubbles in epoxy?

Thanks for that pic.   

To my eye it looks like the underlying plywood edge wasn’t rounded enought to allow the wet-out fiberglass to make the change in direction. What happens is the ‘glass tries to flatten out just where it encounters the middle of the joint edge, forcing the ‘glass on either side to pull away from the surface underneath.

Two ways to mediate the risk: either round the edge more (where you might have 3/32” or 1/8” radius you might try 3/16”) or by using something to pull the ‘glass taut over the joint to keep it from lifting, say by using tape on the un-wet-out edge 2 or 3” from the joint... the edge that’ll get trimmed off anyway as you progress.

Or use lighter weight cloth where possible.

Same thing can happen on inside joints where fillets aren’t shaped with a large enough radius to allow wet-out ‘glass to follow the underlying surface.



RE: Bubbles in epoxy?

Hi Dave, 

took a look at the picture.  spclark pretty much describes it.  i wouldn't categorize this so much as bubbles as the cloth pulled up from the corner and never got fully wet out.  it tends to happen around sharp corners and is not fatal and happens to even experienced builders.

step one is to sand off any glass that did not get wet out.  it will flake off like the skin above a blister....and sand it back into good glass/epoxy.

the next step, after you have done that is to clean up the area for all the dust that collected in the little divots.  i find a toothbrush soaked in denatured alcohol a great way to clean up these little sections.  just brush it in....if it is clear when it is wet....you are in good shape.  if its still white....you probably have a bit more sanding to do.

once you have cleaned it up, then just a little glass patch over the unglassed okoume....as spclark describes....sometimes a lttle tape to hold it down tight helps....then once that cures, just fare it in with a litle sanding block.

my only other advice is don't go crazy on the sanding and sand into the wood.  you are just trying to knock off the glass that didn't get wet out properly and patch it.

all the best, 



RE: Bubbles in epoxy?

Picture without the annoying ads that capture your mouse.

Once thing to keep in mind is that any of the fixes mentioned above are great when the glass is there to protect the wood, but if it's part of a structural joint then the wood really should be rounded to meet the glass layer's minimum bend radius. Forcing it to make a tighter bend folds the fibers and results in a stress concentration, something that you don't want in a structural feature.

Have fun,



RE: Bubbles in epoxy?

Laszlo makes a good point on the structural vs. cosmetic aspects of situations like this. ‘Glass is there for a couple of reasons, the most important being structural integrity where a joint gets stressed routinely as at a deck/hull seam.

Not being familiar with the WD construction, is there a sheer clamp adding strength to this joint under the plywood? That’d add a significant measure of strength to what otherwise may be a low-area glue joint. That ‘glass+epoxy sheathing the outside ought to be the best you’re capable of for long-term reliability. 

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