Top Coating Over An Existing Finish with Lacquer
Applying a fresh coat of lacquer over an existing aged finish can be done but it is a risky concept. In fact, it is one of the riskiest operations in finishing. We do not recommend it be attempted.
- Lacquer over lacquer is feasible, but lacquer over other finishes like varnish is simply not recommended.
- It can result in any number of difficulties, including blistering, wrinkling, orange peel, poor bonding, and in the case of paint, color bleed-through.
There are three primary reasons for failure:
- The existing finish isn’t clean.
- The existing finish is dull.
- The solvent being used in the top coating attacks the existing coating. — Lacquer solvent attacks varnish.
The existing finish must be absolutely clean for a fresh coat of finish to bond well.
- Dust or other dirt can be wiped off the surface or washed off with water or soap and water.
- Grease is common in kitchens, and can be removed with strong detergent, TSP, household ammonia and water, or petroleum-distillate solvent such as mineral spirits or naphtha.
- Wax is a care product often applied to furniture and can be removed with petroleum-distillate solvent without damaging the finish underneath.
- Silicone is oil found in furniture polishes and is difficult to totally remove from a surface because it gets into the cracks and into the wood through these cracks. Silicone causes fish-eye because it is so slick and looks like the finish is flowing away from areas with silicone. To a large degree silicone can be removed by washing with a petroleum-distillate solvent. In bad cases, applying a coat of shellac can block off the silicone residue fairly successfully. There always remains the risk that some silicone remains behind to cause problems.
All coatings have difficulty flowing out over a glossy surface and “wetting” it well enough to establish a good bond, so it is wise to scuff an old surface before recoating. Scuffing also helps with the cleaning by physically removing the dirty top layer.
A solvent is the thinner for a finish, such as mineral spirits for oil paint. The solvent that causes problems is lacquer thinner, which is included in nitrocellulose lacquer, CAB-acrylic lacquer and catalyzed lacquer. In fact, any finish that contains lacquer thinner or is thinned with lacquer thinner or with any of the active solvents in lacquer thinner, such as acetone, has the possibility of attacking an existing coating and causing finishing defects.
Lacquer retarders, that evaporate more slowly than standard lacquer thinners, are more problematic because they remain in contact with the existing coating for a longer time. So “brushing lacquers” are more risky to use because they are made with lacquer retarder.
So, if you want to apply a product containing lacquer thinner, the secret is to spray a thin or relatively dry coat, so that it does not have time to cause damage before drying and sealing the prior finish.
Apply a couple of light coats of finish and let them dry fairly thoroughly before applying a wet coat.
Avoid lacquer retarder, even if this means waiting for drier or cooler weather. Don’t use brushing lacquer.
Apply a barrier coat of shellac between the existing coating and the coating containing lacquer thinner. Shellac is thinned with alcohol, which rarely cause any existing coating to blister or wrinkle.
Based on an article by Bob Flexner in Woodshop News July 2003. We always appreciate his thoughtful articles.
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