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It looks to me like the "crater" is in the epoxy, not the FP. If you apply FP over epoxy too soon it will absorb the solvent and swell, except in areas where you have sanded through the epoxy (or washed it off with the solvent) where there is nothing to swell, so you see the crater. I think you'd see any attempted "fill" using FP/pumice etc. because I think you have a lack of epoxy beneath and my guess is that it will ultimately show (to you at least!).

So, before you have too much time invested in the FP, I'd be washing it off with DNA, letting it dry well and then re-doing the epoxy fill (just on the back, unless you have issues on the sides as well). Give the epoxy at least 24 hours to cure before hitting it with a solvent and start very dry when you apply the FP. If the epoxy is a continuous thin film you have to be careful not to locally remove parts of it.

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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

Trevor's thoughts begs the question as to whether you sanded back to wood before starting the shellac process, and how long you waited before starting to FP? Is this crater in the area where you were trying to fill the pores with pumice? That's what I'd do if I noticed unfilled pores after I started polishing.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Kathy--

You are a few miles from my shop. I do French polish finishing for several builders, including Ervin Somogyi. My friend, Haiying Jiang, does the French polish work for Hill Guitar's "Signature" line. If you'd like to see what we do, give me a p.m. I'm always happy to talk guitars.

The purist in me has to ask: epoxy pore fill--why?

Trevor: Thanks for the additional insights. Maybe the safest thing for me to do, then, is what you have suggested. Definitely couldn't hurt. Would it be better to sand back to wood, instead of level-sanding the Z-poxy?

Waddy: I did not sand back to wood --- I just level-sanded the Z-poxy. I waited two days before starting the process. And, yes, as I previously mentioned, the crater is only in the area I was trying to fill the pores with pumice.....

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Kathy Matsushita
San Jose, CA USA
http://theamateurluthier.com
Facebook: https://www.facebook.com/kathy.matsushita

Thanks for the offer...I'll keep it in mind. As to your question about the Z-poxy...it's just because I've been quite used to using it, and it has worked well for me so far. Of course, it's been under a different kind of finish (EM6000). But I'd seen several mentions, here and there, of people using it successfully under French polish, so I figured it was okay.

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Kathy Matsushita
San Jose, CA USA
http://theamateurluthier.com
Facebook: https://www.facebook.com/kathy.matsushita

Personally, I'd be inclined just to wash off the FP, light sand the existing Z-poxy then apply more Z-poxy and level sand when cured; but I've never used Z-poxy.

My normal fill is WEST 105/207, level sanded then nitro over, and I see what you photographed if I sand through then hit it with nitro too early, straight onto the epoxy. Well cured, no sand throughs, I get a dead level finish which stays level.

For a FP finish on B&S (most tops are FP'd) I don't fill with epoxy, because I want to call it French Polished, which, to me, doesn't have anything to do with epoxy.

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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

Never tried it, and I don't doubt you. However,

Solvent, abrasive, rubbing; a recipe to get what Kathy photographed, I'd say, due to the reasons I gave before. Also, it usually looks nothing like as bad once the solvent has sweated off.

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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

Filippo: I've seen photos of your instruments, and they're lovely! (And I drool over your workshop; it's so organized and NEAT!)
When you use Z-poxy under French polish, how do you do it? Any tips for me? Do you sand back to bare wood, or do you just level the Z-poxy?

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Kathy Matsushita
San Jose, CA USA
http://theamateurluthier.com
Facebook: https://www.facebook.com/kathy.matsushita

+1

Regardless of what caused that depression, it can be leveled with pumice and shellac and patience. Or you can take it down to bare wood and start over, which might be faster.

Well, I waited a couple of days, to let the back FP finish harden some more...Then, today, I spent about an hour carefully sanding down the area around the "crater". (I figured that if the sanding didn't work well, I still had the option of removing the 4 coats of French polish with alcohol down to the Z-poxy.) I used the walnut oil I'm using for the FP, and 1800-grit Micromesh wrapped around an Artgum eraser (I like those for sanding). I just took it really slowly and carefully and lightly, first on the damaged area, then on the entire back surface. Then, after wiping off all the oil and cleaning the area with naphtha, I did a French polish session on the back.

It worked! Thanks, everyone, for all your helpful suggestions and insights!

Here it is (the same area where the "crater" was):

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Kathy Matsushita
San Jose, CA USA
http://theamateurluthier.com
Facebook: https://www.facebook.com/kathy.matsushita

Nice save!

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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

Thanks, Trevor! And thanks for all the insights and help!

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Kathy Matsushita
San Jose, CA USA
http://theamateurluthier.com
Facebook: https://www.facebook.com/kathy.matsushita

Good job Kathy! Knew it would work.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Looks great!

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Jim Kirby
kirby@udel.edu

THANKS, ALL!!! I am so grateful that, whenever I run into a building problem, I can come here and ask for help! I usually get quite a few different kinds of responses and suggestions, but they all contribute to giving me better insights that I may not have thought of on my own. It always helps to know that, if one approach doesn't work in this particular case, another might. Anyway, thanks again for your quick and helpful responses --- they all have helped in boosting my education and confidence!

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Kathy Matsushita
San Jose, CA USA
http://theamateurluthier.com
Facebook: https://www.facebook.com/kathy.matsushita

It's just my view. FP, to me, is a process which has certain expectations about it, due to its tradition. I just don't feel comfortable calling something with an epoxy fill FP'd. If I used an epoxy fill then applied shellac, by pad or spray, I'd call it a shellac finish. I'm hardly a traditionalist and I've no particular axe to grind here, it's just what I (and Eric, too, it seems) feel about it.

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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

Sort of like being comfortable using shellac that has been plasticized vs the pure product. Not saying there's anything wrong with it, and I'm planning to try it myself, but it's certainly not "traditional"!

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

I'm not sure what "pure" shellac is. Bleached, de-waxed etc. etc. and a variety of hardeners (copol, sandarac) and plasticisers (the natural wax, mastic) plus different carrier solvents (methanol, ethanol with varying degrees of water) have all been part of the mix for hundreds of years. The French Polishing process is as varied as there are practitioners. So what is tradition? It's just my personal view that it doesn't involve epoxy.

Anecdote time: I recall a court case once where a vintage guitar had been re-sprayed and sold as "original finish". The purchaser sussed it, took the trader to court and lost, because the guitar still had its original finish, although covered by more finish. Barely relevant, but it's interesting how things can be interpreted. Apparently, what happened then was that, as the new finish hadn't adhered particularly well, the owner had a luthier painstakingly pick off all the new finish, touch up the old finish and the guitar was sold on at a profit as "original condition", because "touch-up" is "allowed". Makes you wonder.

Sorry, Kathy, for hijacking your thread!

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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

...and (rightly or wrongly) many people regard the grain fill as an integral part of the traditional French Polishing process. Others don't.

Here's another anecdote, again barely relevant. I had a guy playing one of my classical guitars, which he was very interested in and he loved the sound and playability. He asked me what I thought about tuning tops, back etc., so I gave him a run-down. I told him about adding side mass and the beneficial effect it has. He asked me what I used for mass. I said "Steel, usually". That put him right off the guitar. He only wanted natural materials, no metal, in a guitar of his. I said "What about the strings and tuners?" Answer: "That's different".

The thing is, in a selection process, any criteria will do. It doesn't have to be rational for a lot of people. Which can be problematical for people like me and likely you, too, and is why I have a view on what I call French Polish, which largely aligns with what the market I serve expects.

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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