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Tiny little pinholes in the finish
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=22149		 Page 1 of 1
Author:  Dave Livermore [ Mon Apr 27, 2009 12:57 pm ]
Post subject:  Tiny little pinholes in the finish
I am curious if anyone else has had issues with a mottled final finish surfaced caused by what I can only describe as tiny little pinholes all over.
My assumption is that this is a spraying/user error with the hplv gun and not a result of materials, but don't know for certain.

The finish is Target USL. Sealer was lacquer mixed from dewaxed flakes.
I've done several instruments with this, but this is the first time this particular issue has occured.

I don't know that I'd call them fish eyes, as they are nearly microscopic. But they occur across the lower bout on the top and in small pockets on the back. I didn't note any on the sides.
On one of the instruments, I did sand through them and leveled without issue.
I just reshot the other guitar to try and spray over them in the hopes they would fill in, but the little holes are present again in different areas.


Please let me know if anyone knows what the cause may be.

Thanks

Dave
Author:  Jimmy Caldwell [ Mon Apr 27, 2009 1:39 pm ]
Post subject:  Re: Tiny little pinholes in the finish
Most likely contamination of some sort. If that's what it is, unfortunately, there's no easy fix I know of. Probably would require stripping the affected area down to the wood.
Author:  Dave Livermore [ Mon Apr 27, 2009 1:48 pm ]
Post subject:  Re: Tiny little pinholes in the finish
Is it possible it is from a curing finish that was applied very thick and then gased off in a way that caused the pores?

This particular top has been giving me fits for weeks. I sanded through to bare wood in one teeny tiny spot and nursed it back by padding on a little finish at a time with a cotton ball. USL is forgiving enough that this could work, but my patience isn't. I sanded through adjacent to the affected area and threw in the towell on saturday. So I taped off the back and sides and set up my spray equipment again and sprayed three more sessions an hour apart each. On the very last one, I thought to myself "hey, why isn't the compressor kicking in?" then I realized I was spraying with a compressor that was turned off and only about 30psi going through the gun. So the final coat was quite thick. I cured it horizontally with a fan on it instead of hanging it to prevent any runs from forming. I didn't think that could have been the culprit, but it is a possible suspect.
Author:  Tim McKnight [ Mon Apr 27, 2009 2:00 pm ]
Post subject:  Re: Tiny little pinholes in the finish
How long are you waiting between coats? How long did you wait before you finished over the sealer.? Solvents need time to off gas between EACH coat or they will be trapped and will eventually fight their way to the surface and leave pin holes as you described.
Author:  Dave Livermore [ Mon Apr 27, 2009 2:20 pm ]
Post subject:  Re: Tiny little pinholes in the finish
Tim,
The sealer isn't an issue. It was a 1lb cut of shellac and was on there for days before I started spraying.
I sprayed, waited and hour, sprayed, waited and hour, sprayed, waited three days, then leveled off and repeated.
Author:  Mike Collins [ Mon Apr 27, 2009 2:54 pm ]
Post subject:  Re: Tiny little pinholes in the finish
[
Dave Livermore wrote:
it possible it is from a curing finish that was applied very thick and then gased off in a way that caused the pores?


Yes sir!
As Tim stated the thinner HAS to find a way out !!

Thin coats ,laid down over 3-4 hours apart works great.
Heavy coats need at least twice the time to gas off.
Plus recoating should be delayed from the heavy coat to the next coat !!
We all want to see the beauties we made -but the finishes take alot of time .
Darn those factory guitars!!!!!!

Mike
Author:  Cal Maier [ Mon Apr 27, 2009 3:01 pm ]
Post subject:  Re: Tiny little pinholes in the finish
Quote:
Is it possible it is from a curing finish that was applied very thick and then gased off in a way that caused the pores?

Absolutely. Your coats should be thin but not too dry. The last coat can be a bit wet, but not too heavy. The pinholes are gases pushing up from previous coats.

Cal

Edit: Mike beat me to it.
Author:  Mike_P [ Mon Apr 27, 2009 5:58 pm ]
Post subject:  Re: Tiny little pinholes in the finish
trapped solvents will though....I once layered 15 heavy coats of automotive lacquer on a bar top and besides the solvent pops (which looked exactly like what has been described here) there were also very tiny bubbles deep in the finish...the culprit?, not letting each coat flash long enough before I added another layer...I was given improper instructions on the application of the product from the store I acquired it at...once I got through to a PPG factory rep. things were cleared up....what about those bubbles you say?, well I ended up falling asleep after staying up for a very long time wet sanding and polishing that bar and had 1,000 watts of halogens pointed at the top to help harden it up for delivery the next day...note I said fell asleep (actually, passed out) gaah I awoke 4 hours later and the finish in the area the lamps were focused was destroyed, which required the stripping of the top and doing it again...I allowed at least 1.5 hours between coats the second time, and only did 5 coats at one session and allowed at least 5 days between those sessions...

the thing is, the product mentioned in this thread is waterborne, right?

are the solvents in Target USL harsh enough to cause solvent pops?
Author:  Jimmy Caldwell [ Mon Apr 27, 2009 7:05 pm ]
Post subject:  Re: Tiny little pinholes in the finish
I've sprayed in excess of 20 guitars with USL and have never experienced this problem. I usually spray 5-6 light coats per session, waiting about 45 minutes between coats. I seal my tops with shellac also, however I generally wait 24 hours before I spray the USL. Since shellac is alcohol based, I have a hard time visualizing outgassing as being the source of the problem. I would expect the problem to be visible with each coat, so if they don't appear until later, the diagnosis is more difficult. It still sounds like some sort of contamination to me.
Author:  Dave Livermore [ Mon Apr 27, 2009 8:43 pm ]
Post subject:  Re: Tiny little pinholes in the finish
I just leveled with 400 grit and I think I got through the pinholes.
It started to get that blue hue that USL gets when it is wet, but not cured, so I'm going to let it hang another day.

This was supposed to be the guitar where I finally figured out the whole finishing process and in the end, I'm sure it will be fantastic!

FAILURE IS NOT AN OPTION

This build has been an extremely educational process. (whenever I see this in a post, I read between the lines and know that MANY mishaps were fixed (or not) and those mishaps will hopefully never happen again.)

Unfortunately, I don't know what caused this. I'm thinking it could have been temperature related as even though all materials were warm, the air was in the 40s-60s when sprayed. OR as mentioned something could have been contaminated, but I can't imagine what that contamination would be. This redwood/walnut instrument was built side/side with a Sapele/Sitka and the problems experienced were exponentially higher on this than the other. I think it was cursed.

I'll chalk it up to gremlins.

No wonder one forum calls finishing the Achilles Heel of building.
Author:  Jim_H [ Mon Apr 27, 2009 11:21 pm ]
Post subject:  Re: Tiny little pinholes in the finish
Ahhh gremlins.. thats an easy fix..

Just put up your "No Gremilns" sign!

Image


Sorry.. couldn't resist. :)
Author:  Erik Hauri [ Tue Apr 28, 2009 11:21 am ]
Post subject:  Re: Tiny little pinholes in the finish
I've gotten this using pre-cat conversion varnish - I find that if I apply it too thick, it will skin over (like, in minutes) before the underlying material has had a chance to flash off, resulting in very tiny bubbles that accumulate underneath the skin and aren't visible until I start to level the finish.
Author:  Dennis Leahy [ Tue Apr 28, 2009 8:50 pm ]
Post subject:  Re: Tiny little pinholes in the finish
Todd Stock wrote:
Unless the finish and guitar was heated after finish was applied, outgassing will not cause bubbles.

This was my first thought too. Spray in a cool garage, bring the guitar indoors and put it in a warm cozy closet... and tiny bubbles can appear.

Dennis
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