Bubbles in lacquer on inside corners

[Alan Bienlein]

We have been having some problems lately with the finish bubbling on the inside corners of guitars that are being finished with lacquer. It happens either at a neck joint or at the bridge on a steel string guitar. We are at a loss as we have tried thinning the lacquer more and we also have tried retarder thinking that might help. Here are a few pictures so you can see what I'm talking about.

The guitars are sealed with de-waxed shellac from flakes mixed with everclear and the lacquer is instrument lacquer from Cardinal.

The first is of a semi hollow guitar where the neck joins to the body.


This is a steel string and the bubbling is happening on the neck side of the bridge.
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[Wayne Cannon]

I have no direct thoughts; but here's one thought. When finishing a wood floor, if the temperature rises after laying down the finish, air expands as it warms, and effectively "blows bubbles" in the finish as it escapes from the wood beneath the finish. Our last floor finisher hadn't heard this, and we got tiny bubbles in the finish -- several dozen per room, usually at board ends, and usually single bubbles (three or four at most, not the "frothing" you seem to be seeing).

[Mike Dowell]

What are you using to apply the lacquer? That, to me, just looks like dust, fuzz, and some air turbulence(if you're spraying). If you can tell me a little bit more about how you are applying this, I can likely help.

[Alan Bienlein]

Its a gravity fed spray gun. Everything is wiped and blown off with and oil less compressor from california air. The process is to seal with de-waxed shellac. From there it gets about 2 coats of clear lacquer that is sanded flat and then the next coat of lacquer is tinted to the desired color. From there the final coats of lacquer are applied. This last step is when the bubbles start to appear. I can find out the make of the gun tomorrow.

[Mike Dowell]

Seems like it's turbulence from spraying inside corners. I get that when I spray the insides of drawers. Can you turn your sprayer down any? When I do inside corners, many times I have to turn my sprayer down to a "soft mist". Otherwise, I'd carefully sand that smooth, and touch it up with a rattle can as that's a very soft spray.

[Wayne Lomman]

Alan, it is caused by out gassing by some change in your process, given that it hasn't always happened. You have tried retarder and extra thinning which is the right way to address the problem initially. I think you need to go back to the source of the problem. Have you changed the glue? Is the glue completely filling the joint? Is the glue temperature right?

To get out of trouble now, Wayne Cannon's flooring example is relevant. Make sure the instrument is warm, shut off your heating and spray a coat with the temperature falling. I had to do this spraying a concrete floor once - well, 10 acres of concrete actually - and we could only spray late afternoon and night when the temperature was falling otherwise it looked like the worst case of chicken pox ever. Cheers

[Alan Bienlein]

Today he gave the gun a good cleaning and so far problem has went away. Looks like it might not have been atomizing the lacquer fine enough trapping air in it.