Official Luthiers Forum!

Mario, great tip on the reverse sanding, thanks. When you noted using stuff like french polish, do you go the exact same way as using shellac. This sounds like a really neat thing to do, and I may want to try it out. I have a couple of old bodies that I picked up from another guy for testing stuff. I may try french polish with the Oxford. Have to get everything moved and unpacked, but once done, will do it and post results. Thanks again for the tips Mario

John...The temp was between 68* to 70*. Humidity was between 35% to 40%. After shooting, I had some orange-peeling which leveled fine with RO sanding and block sanding. FWIW, Target recommends thinning with 5% water to reduce orange peeling. The blotching wasn't visible until I began the wet sanding process.

Mario...If I have time today, I'll call Jeff and post his thought process and diagnosis. I'm not sure I'll remove the finish, but it's nice to know that it can be removed with paint remover. I might try that on the tongue and bridge areas. Nice tip on reverse sanding!

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JJ
Napa, CA
http://www.DonohueGuitars.com

Yea, Mario, I'll add my thanks for the reverse sanding tip. I'll give it a try and report back.

Thanks,
BruceH

I spoke with Jeff Weiss at Target. Here's the line of questioning and his conclusion:

...Could be the Z-poxy on the back & sides. 9k only forms a mechanical bond on epoxy and I might be seeing whitness lines. (BTW, he recommended shellac over Zpoxy so that it will form a chemical bond). But why then does it occur on the spruce top sealed with shellac sanding sealer?...Oh! ok, then...

...Sounds like a witness mark, like it's not bonding to the substrate...or...

How many coats did I shoot?...four heavy coats...that's not enough; I may have sanded through and what I'm seeing is the sealing substrate; rough-sand back with 600 grit and shoot more.

So, that's where I stand. Jeff Weiss seems quite knowledgeable and very willing to help. What's frustrating is not being able to even describe the blotching accurately...I need to have someone knowledgeable actually look at it. Perhaps I'm being too picky... several friends can't even see it until I angle it in the right light and point it out.

Anyway...I'm going to leave it as is. I actually believe I have a sufficient coating and I'm not convinced that I've actually sanded through to the sealer. But now I'm willing to try it again because I'm so close.

If I were to start a new guitar, here's what I'd do:

...lay down a coat of shellac on all surfaces before shooting 9k.
...prep sand with 320
...shoot 2 coats of 9k < 60 minutes apart
...rough sand @320 after 2 days
...shoot coats 3 & 4
...rough sand @320 after 2 days
...shoot coats 5 & 6
...level sand from 320 to 400
...wait 2-3 weeks
...complete wet sanding from 400 conventional grit through 12,000 MM
...wait 1-2 weeks
...buff with Menzerna Fine & Extra Fine
...cross my fingers & toes!

BTW Bruce, he said that a foam brush is a perfectly acceptable method of application as you can attest...you just need a good hand to apply it evenly.

Lance et al...Now I know why you guys send your guitars out for finishing! Another 5-10 guitars and I might have this thing figured out!

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JJ
Napa, CA
http://www.DonohueGuitars.com

JJ,
Thanks for the follow up. FYI - I'm using Z-Poxy also and am not having the same problem as you.
BruceH

JJ, where did you get the 9000? I'm wondering if this may be the difference. I would think buying direct from Target Coatings might be the best route to ensure the most recent formulation. I'd ask for the most recent version (my PSL can has a name followed by v3 so you can tell the formulation has changed).

Mario, what problems can expect with PSL? Do you still recommend shooting 9000 as finishing coats over PSL?

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Don Atwood
Arlington, VA

I always pad or shoot a shellac sealer over the Zpoxy to insure a good bond. No matter what my finial finish is. But of course I only do two finishes right now, French polish or KTM-9. I have not had the chance to try Oxford 9K yet. I'm still working on perfecting my KTM-9 process.

PSl? That muist be old stuff, as they don't make any more, do they? I think USL replaced it...

JJ, here's a sure fire way to tell if you cut through the top coats and need more:

wipe a wet coat of oil, walnut or even olive oil from the kitchen, on the area. Now, if the blotchiness dissapeared, you know you cut through and are seeing the sealer coats. Shoot more, and go forth. If the area still shows the imperfection, it's not the surface, but further in.

When done, wipe the area with naptha a few times to remove all the oil after you've checked it.

several friends can't even see it until I angle it in the right light and point it out.

If the areas are that hard to find, you may be best off to leave it; once yuo have some greasy fingerprints on it <g>, it will be al but invisible. For guitar #2, this sounds like you have done an excellent job so far! Of course, being so close to spot on perfect, it is tempting to keep trying until it is. Tough call, but....

....One of the wisest old shop teachers I ever learned anything from remarked that we only learn when we have trouble, and that someone who flukes through every task perfectly the first time through, will fail miserably in life, because when the inevitable happens, he'll have nothing to fall back on. In that way of thinking, you are doing great; you have a peerfectly reasonable finish done if you choose to leave it as-is and move on, and have learned a good bit in the process, or you can keep trying to trouble shoot, and see if you can't learn some more. Cool!

Don...I bought a quart directly from Target and it sat unopened for 3-4 months before using it.

Mario...Thanks for the oil test tip. I tried it with olive oil. The blotch was still visible through the oil. So I guess that confirms my suspicion that the finish wasn't sanded through. Before doing the test I made up my mind that if it did reveal sealer that I would plod on with more spraying. Since it didn't reveal sand-through, I'm going to stop here.

Thanks for walking all of us through this and for the kind words of encouragement. Even though this finish looks better than my Martin and my buddies think I'm crazy, I know I'll do an even better job next time. On the twin parlors I'm completing, I'm going to finish one with nitro and the other with 9k.

I'm looking forward to posting the final, final pictures of this epic effort that has spanned a year to date...probably in another month (with my crazy schedule.   

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JJ
Napa, CA
http://www.DonohueGuitars.com

At any rate JJ it sure is a fine looking box. I think the binding and rosette are very tasteful and I love the white soundhole binding. What is that anyway?

John

Is the blotchyness on just the top or on the back and sides too? The reason I ask is that I had a similar problem on a top once with a different finsh and it turned out to be runout in the wood. I didn't notice it while I was sanding the wood or during the early coats. It only became obvious when I tried shining it up. The problem was visible all along but I lacked the experiance to see it. I sanded down all the way to the wood and the problem didn't go away! There didn't seem to be any fix for it so I just rebuilt the finsh and went on. If I was going to sell the instrument I would have replaced the top.

John...Thanks...the soundhole is actually not bound but the inner purfling is just B/W/B. The binding around the sides is ebony. The rosette and edge abalam is the stuff from your Duke of Pearl group buy a long time ago. You've probably made 3 or 4 guitars since that time!

Mike...The blotch is on all surfaces, randomly scattered in patches from 1/8" to 1" in non-uniform shapes and in no way is related to runout. This one, nor any of my guitars are for sale. This particular one is a "legacy guitar" for the 2nd of my 3 sons...along with a 150 page journal describing it's build progress and all the trials and tribulations that went into the effort. Hopefully, they'll appreciate how important it is to develop a strong work ethic and how to approach problem-solving.

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JJ
Napa, CA
http://www.DonohueGuitars.com