How long after shellac coat, can I add lacquer?

[Jerry Ryan]

I'm still working on my busted Guild GAD50E project with a poly finish, used hide glue & CA to fix cracks, scraped and sanded w/ 320 then 800. Sprayed 2 layers of 1# shellac. Its been over 24hrs now, but I was advised to wait a week before sanding the shellac and spraying w/ nitro lacquer.

Any advise?

[Bob Webster]

Hi, Jerry. Maybe the forum membership is playing a cruel joke on you. If we all wait long enough to reply, the sprayed shellac will have dried sufficiently. Truthfully, a couple of days for drying down 1# cut would be sufficient unless it's over rosewood, in which case it might take it up to a week to be sandable without some lubricant. Don't get too aggressive with the sanding because there won't be that much shellac on the surface; just not as much solids as a sanding sealer. You just want to roughen the surface to give the nitro a chance to stick.

Cheers,
Bob

[Jerry Ryan]

Thanks! I am pleased with how this project is progressing.

Bob, what kind of finishes should you not use CA to bond/fill cracks?
It works great with this Guild poly... questionable with a lacquer finish on a Seagull, finished that project with multi-layers of nitro.

[Bob Webster]

Any finish with acetone or MEK in it is going to be a potential problem because even kicked off (polymerized) CA is going to be soluble in it. It's hard to say for sure how bad the problem would be because it would depend in part on how long the exposure to the solvent would be (i.e. how quick does the finish dry down). I'm speaking, of course, on the plane of the hypothetical because I don't have any direct experience with this specific issue. What sort of solvents are in the poly finish, by the way?

Nothing beats hard data. The way to get it is to make a dent in a scrap of rosewood or whatever, fill the dent with CA, level it, and then apply the candidate finish. If you see significant sinking of the CA fill, then that's a fill and finish combo that isn't going to work out.

Bob

[Frank Ford]

I don't spray shellac for a variety of reasons, not the least of which is gun cleanup. I wipe it on, typically one coat only, and don't hesitate to overcoat with lacquer a few minutes later. My wiped coat is pretty thin, and I don't sand it at all, preferring to level the lacquer when the time comes.