problem with pinholes in lacquer finish on Mahogany - created 11-07-2007

Gleason, Robin - 11/07/2007.18:33:30

I am making a hollow body, carved top and back electric ukulele. The top is curly Koa and has no finish issues, but the back is mahogany and is having issues. The instrument was finish sanded and grain raised two times before sealing with two coats of vinyl lacquer through HVLP system. No problems at this point. Then after a light leveling, I started spraying 3:1 McFadden's and pin holes appeared. Sanded it down, and re-sprayed with fish-eye remover, more retarder, and lighter coats. The Pinholes came back again. Sanded it down and sprayed vinyl sealer and the pin holes are there again. Any suggestions?

Robin, is the grain filled completely?

Sounds like pore filler was not used and the pores were bridged over with the vinyl sealer, which allowed the top coats to sink into the pores. Using fish-eye remover was not necessary and will now pose its own significant set of problems. I am not sure what to recommend as a remedy other than stripping and starting from scratch using a proper pore filler.

I neglected to mention that I applied 2 coats of CrystaLac pore filler. By spraying with coats of thinner and I have been able to cut down the pinholes to just a few, but I may still need to start over (and thoroughly clean my gun to get rid of the fish-eye crap). I also found info stating that spraying lacquer on a surface hotter than the lacquer can cause this issue. I keep my gun and the instrument in a hot box between sprayings. Now I have tried just putting the instrument in a couple of minutes prior to each spray and leaving it for a few minutes after each spray (elliminates fogging). My spray room is not dehumidified, but my work room is, and the outside environment is extremely humid (rains daily). I will decide today, whether to start over or not and let everyone know the outcome. Thanks for the input.

I know about the humidity. Then again, I'd rather spray on a high humid day than glue up (probably why your build room is controlled as well), and I've never had blushing issues with McFadden's, either shooting straight, 10% (which is what I spray the first 6 at), or 50% (for the final coats).

Since you're using a hot box, you're probably spraying onto a fairly dry (and warm) surface between coats, which may possibly represent spraying onto a "new" surface. Have you tried spraying a tack coat first? I don't think its really necessary, but I usually do it on the very first coat, especially when I used to spray more than one instrument at a time.

You also stated that you're using an HVLP system (so I assume its not a conversion gun), which by its very own nature sprays warm air as well. This can be good or bad - warm air through the gun is supposed to flow and dry the material better, but if you're doing that onto an already warm surface, I've not experienced that and what would result.

Also, you may want to call McFadden's, they're more than willing to help. I spoke to Peter just yesterday, and he schooled me on using their Poly - looking forward to it!

I had this problem once with an ash body and although I was very thorough in my prep, I had about six pin holes that just refused to go away. I finally smoted them with a couple applications of thin CA dropped into the pin holes and moved on. I wasted a lot of time with these little buggers when it was just a matter of ten minutes to solve the problem with CA and cutting it back with a single edge razorblade. In my case, I just chalked it up to some sort of contaminent such as silicone.