Finishing over unknown finish

[Alan]

I am currently working on an old Kasuga classical guitar which had a broken headstock. I have re-glued the break and intend to strengthen the head with a carbon fibre rod then glue on a new sandwich of head veneers on the face and a single veneer on the back. So far I have removed the finish from the whole of the head and neck back to the body.
The finish on the body is OK with a few blemishes/scratches etc. My question is, can I spray over the existing finish on the body with lacquer after light sanding? I tested the existing finish with acetone and it doesn't seem to affect it so it may be a poly finish - though I thought this guitar was old enough to pre-date poly finishes. If the finish is poly, will lacquer go over it without causing wrinkling etc.

[Crafty Fox]

Alan, poly finish has been around for quite a while now; I've got a 70's Strat which has a poly finish. If in doubt seal with shellac?

[ozwood]

Robbie just did a Tute that may be useful.

viewtopic.php?f=1&t=6519

Cheers,

[Alan]

Thanks Paul - I had viewed that video but it still leaves you up in the air as to whether lacquer will go over a Poly finish.
I suspect that either the lacquer will "wrinkle" the existing finish or that it will not bond, sitting on top and chipping off readily.
I could sand all of the finish off the body (time consuming) but if the back and sides are laminated, I run the risk of sanding though the face veneer.

[Nick]

I had a similar repair to a 70's era Kasuga steel string and the existing finish smelt like poly (has quite a distinctive 'plastic-y' odour when sanding) and behaved as such at refinish time. If it's passed the Acetone test then I'd say it's all good to go ahead with any finish you want to apply , just make sure you sand the existing finish (320 grit) to a consistent dull finish, no shiny spots! to allow the new lacquer something to key on to.

[old_picker]

polyester is a very tough finish - i doubt spraying lacquer over it will touch it
if it were me i would not hesitate once i had roughed up the existing finish