Finishing Problem / Challenge

[Graham Long]

Hi
I've made a few Guitars and finished them all with nitro-cellulous laquere.
All has been good except for the soundboards on two of them. there appears to lines of tiny little bubbles along the hard grain lines.
I've tried sanding the finish off and respaying - still there. I've tried sanding off and cleaning the surface with thiners and/or acetone - no good they still come back.
Can anyone help??

[kiwigeo]

What sort of wood on the tops? What is the preparation schedule prior to spraying?

[Dominic]

Chuckie, in Dan Erlewines finishing book (great book, particularly for colouring) he suggests that using too thick lacquer can result in bubbles trapped under the finish. You can spray thinner on the surface to float them out. And using a mix with retarder and more thinner to slow skinning over to allow bubbles to float away themselves. But the message is that spraying too thick results in bubbles.
But perhaps it is gassing out of the wood itself and is not being trapped by the spray. Are you using spruce for the top or some other wood?
Cheers
Dom

[simso]

It sounds like your not sealing the grain off, or the sealer your using for the grain is reacting to the top coat.

[Graham Long]

Hi All,
I've been sanding the tops, which have all been spruce, down to 320 grit. The earlier one was first sprayed with a very weak lacquere/thinnners mix 20-30%, the later one was first sprayed with sanding sealer, again a weak mix. For want of a better description i've called them bubbles, but they are probably more like very small pin holes in the lacquere that follow the dark hard wood lines.
Cheers

[Kim]

Gday Chuckie, welcome to the ANZLF.

Just a guess but it sounds to me like the winter grain (dark wood) could possibly be gassing off and reacting with the lacquer. I suppose this could happen if the wood was shipped to you before it was completely dry. If that is the case then one would expect the problem would reveal itself in the winter grain as it is much more dense than summer grain and therefore would give up moisture more slowly. What sort of spruce are we talking here, was it all the same species and from the same vendor???

Cheers

Kim

[kiwigeo]

For want of a better description i've called them bubbles, but they are probably more like very small pin holes in the lacquere that follow the dark hard wood lines.
Cheers

IMO youre looking at wood fibres (nibs) that haven't been sanded back. I had same problem french polishing some very furry Engleman spruce on one of my classicals. On my top the nibs were breaking through the shellac and running in lines along the darker wood. They looked like tiny bubbles at first glance. I took the top back to bare wood and then applied an egg white size to stiffen the wood fibres before sanding back to 320 grit. Under normal circumstances Id certainly damp down the wood after sanding back and go over the top again with the 320 grit paper a couple of times before starting the finish.