Lacquer over Urethane on Site

A builder gets advice for using a shellac inter-coat to isolate a lacquer top coat from already-applied urethane. October 28, 2008

I'm a contractor with various guys coming and going over the span of this 4 year custom house we're building. Several areas such as the window liners/casing were brushed with urethane. I've taken up lacquering the natural hardwoods. I've heard a lot about compatibility issues regarding lacquer/urethane. I'm not clear if I could spray light coats of lacquer over the urethane or there is a sealer/primer available to use. I'm using Campbell catalyzed lacquer over a generic urethane that was picked up at the paint store.

Forum Responses
(Finishing Forum)
From contributor M:
If you are talking about spraying lacquer over the polyurethane and the poly is well aged, you might be able to get away with it by spraying light well-thinned coats of the lacquer. However, the poly might lift. The safest thing to do is to use dewaxed shellac over the poly and then you can go to the lacquer without worry.

From contributor A:
I would assume that the aged oil based poly has good resistance to the lacquer thinner. You could take a rag dampened with lacquer thinner and test a section.

From the original questioner:
The urethane went on perhaps 6 months ago. I'll try the shellac idea. Can I spray this, and how long do I need to wait before going into lacquer? I suppose I'll need to sand it out again.

From contributor M:
You can spray waxless shellac as easy as anything. Get some denatured alcohol (shellac thinner) to thin it for spraying, but it is already fairly thin. The beauty of shellac is that it doesn't bite into anything and nothing bites into it. So it is great for isolating two finishes which would harm each other. But by this same token, the finish prior to it (poly, in your case) will have to be rubbed down a little to give the shellac a mechanical bond since the shellac won't bite into the poly for a chemical bond. Same thing goes for the prep on the lacquer over the shellac.

From contributor G:
Have you tried using vinyl sealer as a bridge between the poly and the lacquer? It works effectively when you go between any nitro lacquer and conversion catalyzed varnishes, or vice-versa.