Official Luthiers Forum!

I apparently buffed out the finish on my mahogany L-00 prematurely. I did the final sanding up to 1000 grit, and it appeared to be completely flat. However, when I buffed it with rubbing compound, the pores re-appeared in the finish on the back only.

I decided to sand it flat again so that I could spray a few more coats on the back. The problem is, now I have hundreds of very tiny white pin holes. I have tried to sand more, I've tried to clean with naptha, but the white spots remain. My best guess is that the rubbing compound is in the pin holes.

Any suggestion on something that may remove the white spots?

Thanks for any help.

James

It sounds to me like it's probably not from buffing too soon. They were probably there all along and you just didn't see them until you buffed. If was sinkage, I don't think the compound would build up in the pores.

If it's nitro, the only solution I'm aware of is to sand them out. Unfortunately that often means going almost all the way back down to the wood.

If it's nitro, they were likely the result of spraying coats too thick. The surface skins over while there is still solvent trying to escape. The skin traps the solvent as bubbles and they turn into pin holes as you sand into them. The way to avoid that is to spray thinner coats and/or add a bit of retarder to the lacquer.

Good luck.

_________________
http://www.chassonguitars.com

Sorry, I should have mentioned that the finish is nitro. I sprayed 3 or 4 coats with at least an hour or more between coats. Then I would let it hang for a few days before scuff sanding and spraying additional coats. I mostly worked on this one on week ends only.

You need 3hrs between coats of nitro.

Most of the time there were at least 3 hrs between coats. After the finish was sanded flat, with no pores showing, I sprayed 2 more final coats and let it hang for 3 weeks before final sanding and buffing.

Are you saying that not waiting 3 hours between a few of the coats could cause the pin holes?

That is what I was afraid I was going to find out. Hello stripper, goodbye pin holes.

Thanks to everyone for the assistance.

James

stripper may not be necessary....

I've had that problem before and if you are using a compound that is paint shop safe (i.e. no silicones) then you won't have any issues with contamination...but you do need to remove the compound from the solvent pop holes...I've successfully used lots of mineral spirits before when I had that issue when I was spraying very thick coats of automotive lacquer on bar tops...

you DON'T need to wait 3 hours between coats, 45 minutes to an hour will suffice if you aren't using lots of retarder.

I think heavy sanding with 220 grit to get to a fully level surface will suffice, clean with mineral spirits, wait a good hour for the spirits to flash off, and start spraying again...

Never heard you have to wait 3 hrs between coats. I always thought nitro was valued for it's fast curing time among other things.

_________________
"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

what did you use to buff it out? Have you determined that it's buffing compound (liquid or dry) in the pores and not simply the sanding dust from the lacquer? Are these pin holes in the finish or wood pores that haven't been filled enough?
Craig

When I learned to spray nitro it was from Michael Hornick of Shantee Guitars (some of the best finishes, bar none). And everyone I've ever known who's sprayed McFaddens has done the same:

1st day, spray 3 coats, 3 hours apart
2nd day, scuff sand the surface to break the surface tension and allow the finish to off gas
Repeat until enough coats are on the guitar, usually about 3 sessions to end up with about a finish film of .004-.006"

Wait a minimum of two weeks, preferably a month. Otherwise you'll get terrible shrinkage and the film will be more prone to imprinting from the case.

McFaddens has always had a lot of plasticizers that require time to off gas, hence the cutting of the film surface and the time between coats.

As for retarder, if I remember correctly, I sprayed with the finish thinned about 20% and I think maybe a capful of butylcellosolve (retarder) in a quart. So not a lot of retarder or thinner.

-- db

45 minutes to an hour between coats of McFadden's has always worked fine for me. But, with the drastic Seagrave price increase we'll see how Mohawk works with the same spray schedule.

_________________
michael propsom
www.propsomguitars.com