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Well I just finished sanding a coat of Epifanes 2 pack polyurethane off of a cedar top
which was already thin enough...very tough and very irritating. I thought it was my spraying technique/mix etc etc
but did a little test on an off cut this am and there it was again a bubbly reaction. put some on the rosewood bridge..no problem.
The Koa Back and sides from the same mix were fine as were the neck and even the walnut rosette
Any body have any idea what is going on?

This is not much help, but...

I've had the odd piece of cedar that I couldn't French polish and usually WRC FPs really well. After three attempts, I shot nitro on it and it was fine. So some cedar contains something that really hates some finishes. I never figured it out further than that.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I'm really pleased to hear that! I've been struggling with a piece of cedar which was rejecting shellac, though OK with oil varnish. I thought it was me (of course, still could have been).

Bizzarely, a previous top from the same board (not the identical length) was no problem at all.

Cedar can be an oily wood. Not the best match with finishes at times. What is your finish schedule? Curious what would happen if you scuffed back and applied another coat.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/