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This is my first try at FP and it was going surprisingly well until the 2nd build session. It's koa, with Z-poxy filler, blond Hock shellac, 1 lb. cut with Bektol. It was applied at about 70 deg, 45% RH. First session was Saturday, second one yesterday, third one today.

Sometime after yesterday's session, I noticed some bumps on a few areas of the back. They weren't there during the session, nor at the end of the session, but appeared later. Today, I sanded lightly with 600 and they went away. I did another session, but more bumps appeared in different places, about 20 minutes after the end of the session. There seems to be no further occurrence after this. They appear to be sliver-shaped blisters. They're not everywhere, only in a few square inches. Otherwise I'm very pleased with the process and the outcome.

Has anybody had this happen? Any ideas for a cause? Thanks for any ideas.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

What I see looks like muneca dust and or lint contamination. If your muneca picks up air born dust and or is sheading lint it will deposit this during bodying sessions. I also see the normal ridging caused by the circular application of the shellac this is no issue

Looks like some type of contamination. What type of oil are you using?

If the areas I marked as shellac ridging are the blisters, as you mentioned. Then there is a surface contamination of some sort.

If this is the case the shellac is bubbled up (blisterd, not aheared to the wood in those areas) Is this what you are getting?

Thanks for the replies, Michael and Richard.

Looking at it closely, it appears that they are small blisters. It looks as if there is a tiny crack in the finish at the peak of the bumps.

I'm using walnut oil. Could that be an issue?

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now known around here as Pat Foster
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http://www.patfosterguitars.com

One thing I'm always careful of when French polishing is to filter both my shellac and oil, I to use walnut oil, which can contain solids. Did you spirit off at the end of the session?

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Colin,

I filtered the shellac, but not the oil. The oil looks very clear and clean to me, but I suppose it could contain transparent solids, could it not? I did spirit off, but if there is contamination of the muneca, I could well be adding to the problem.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

HUmm did you sand the zpoxy back to the wood or did you leave a film of it on the exterior of the wood. I always sand the zpoxy back to the wood the make a 60% zpoxy 40% alcohol mix (this is super thin) and lightly wipe just to even the color. after it cures I sand lightly with 320p. this gives the shellac something to bite on the first couple sessions. This should not be a proble anyway as the shellac should stick well enought to the thin zpoxy, but I do this just to insure the bond.

I have never used anything but alcohol as my solvent either DA or pure grain. The more I look at the photo the more it looks like an adhesion issue. Either contamination, a gassing off issue or possibly a wax issue with the zpoxy. Are the flakesyou used 100% wax free?

100% pure Walnut oil should be no issue. If it does not say either virgin or pure Walnut oil it is possible the bottler added some vegstible oil additives to cut cost.
This can cause adhesion issuesMichaelP38986.5176851852

Michael,

I do think there's some sort of adhesion problem in the affected spots, since it doesn't seem to appear right away, but I'll look closely the next session. Looks like it could be outgassing possibly.

I first used the Z-poxy out of the bottle and applied as thin a coat as I could. I sanded back only far enough to eliminate all the gloss, which was back to wood here and there. I did two more fills, with the epoxy thinned about 20%, with a light sanding with 320 following each fill session.

The Bektol MSDS pdf shows it to contain 81 - 90% ethyl alcohol, and it smells and acts like generic DNA I've used. I thought I saw on the web where Alan Carruth said he uses it, but that's relying on my sometimes shaky memory. The shellac flakes are 100% dewaxed (how can they be sure?) from Ron Hock, so I think I'm OK there. If it is indeed adhesion and not particle contamination from the muneca, then that leaves the oil, the filler, the solvent or my technique/schedule. From what you're saying about additives to the oil, I think first I'll try changing oil, do a session, and see what happens. Then the solvent. I really hope it's not the filler! At least the affected areas are small. I just hope they don't act up later, but this is for the family, so even that isn't so much of a problem.

Thanks for you help.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

What you said about how you processed the fill, I don't think zpoxy is the issue. I use it all the time and have no problems. I would test on scrap but I think I would change your oil first and test. Because if it is vegetable oil in the walnut oil you will need to remove every thing down to the zpoxy. Clean well with nathpha a couple times to be sure to get it all off and hope it did not penetrate the wood any where. MichaelP38986.6376273148