I have several layers of sprayed lacquer on a cabinet in my workshop. Keeping in mind that it may get a little wet from time to time because of its location, can I spray a polyurethane on top of that without a problem of adhesion and durability? I thought I read someplace that shellac and lacquer are a good foundation for almost anything else except penetrating oil.
John Upland, CA
In a perfect world, the best thing to have underneath your polyurethane would be clean bare wood. That being said and needing to deal with the real world, since polyurethane (a type of varnish) relies on mechanically bonding to the previous coat of finish, you need to be sure to sand the lacquer coating before top coating with the poly to ensure the best adhesion of the poly to the lacquer. The "tooth" that the sanding scratches provides gives the poly something to grab onto. A slick smooth finish of lacquer would provide poor adhesion to a poly coating.
The two finishes are different and may have different levels of flexibility and deal with any wood movement differently. Will you see cracking or crazing at some point because of this difference of the film finishes applied? You might or might not. Only time will tell. If you keep the piece in a climate controlled environment, out of direct sources of extreme temperatures & moisture (including heat from direct sunlight), this may keep movement of the film finishes to a minimum and minimize any issues of cracking, crazing or long-term adhesion.
Worst case scenario is that the finish lets go at some point and you have to remove it all and start over, this time using the better choice of finish to meet your needs.
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