Tiny bubbles in my finish

[CGHbuilder86]

I am spraying a nitrocellulose lacquer finish. Tiny bubbles are appearing, and I can’t figure out what is causing the problem. I've never had this problem before, though I haven’t had a great lot of experience. I have an air compressor that has an oil free pump design. I do not have any water traps or filters though, could this be my problem? I also need to add that my spraying conditions are not the greatest. The temp when I was having this trouble was about 84, the humidity about 53%. I have a window unit air conditioner that cools my shop but I can’t run it while I am spraying, because I don’t have a spray booth. I guess I should look into getting one, but anyway if any one knows the answer to my problem I would appreciate a reply.

[Southpa]

I went thru a similar problem this summer. You got too much heat in the guitar. Finish dries too quickly making a "skin" over top and the stuff underneath is still trying to gas off. I was clearcoating a black guitar out on my bench in the back yard. Left it in the sun for only half hour prior to spraying. That was enough to absorb heat resulting in bubbles up to 1/2 cm across. AND, the bubbles went right thru all the previous coats I laid down because new laquer softens old laquer. So I had to sand it all right back to the wood and start over.

[westhemann]

in the book i have it suggests thinning your laquer in order to allow the bubbles to pop out

i also seem to recall something about adding a retarder?

[CGHbuilder86]

Thanks for the help. I’ll try spraying again when it is not so hot. I did use a little retarder in my lacquer, about 15% I think, maybe I should use a little more.

[LGM Guitars]

This is known as solvent popping.

It is usually caused by the heat causing the outer surface to skin over to fast as was mentioned, however with Laquer humidity can also cause it. It doesn't sound like you had excessively high humidity though.

Even though laquer melts back into itself, I would still recommend sanding the finish out and getting rid of the solvent pops before re-coating it. It will give you a more orange peeled look if you don't do this.

Try spraying lighter coats and leave about an hour in between coats if it is hotter than about 74*

edit: Thinning your laquer will create more solvent popping if it's to hot, the bubbles are caused by the solvents escaping and causing the tiny craters (they look almost like pinholes) in the dry outer surface. Thinning laquer causes it to flash off even faster.

[Engineer]

Regardless of the humidity, it could come from moisture. Water is a natural by-product of compressing air. A small investment in an oil/water separator is well worth it.