Recoating with Conversion Varnish over an Old Finish
Adding a more durable topcoat over an existing softer finish can introduce tricky problems. August 26, 2006
Question
A customer has some cherry cabinet doors with some white spots in the clear lacquer finish from moisture exposure. I would like to sand these bad spots out and apply clear conversion varnish. Can I expect any problems with adhesion or compatibility with the old finish?
Forum Responses
(Finishing Forum)
From contributor A:
Yes. Can't you just strip these doors or wash with a strong lacquer thinner and then color match and apply the CV?
From the original questioner:
I want to do as little as possible to get a little bit better moisture protection; just rub out the white areas and re-coat. There isn't any money in this job, it is just a favor to the customer, and I don't want to get into a total strip and refinish job.
From contributor B:
Finishing 101 states that you cannot put a harder finish over a softer one but you can put a softer coating over a harder one.
From contributor E:
Have you tested the existing finish? You say clear lacquer finish but that covers a lot of territory. Does the finish dissolve when you wipe it with lacquer thinner? Does it dissolve right away or do you have to scrub? Test the finish in a low visibility area.
From contributor C:
Shellac is commonly used under several coatings that are harder then the shellac - coatings like lacquers and waterbase coatings. Varnish is much harder then shellac, which is the sealer that is used under varnish. Many finishers also use dewaxed shellac under polyurethane, which also is harder then shellac. I don't think I would use a softer finish over harder finish. I want the more durable coating on the top.
From contributor D:
Do you detect a theme in the answers posted here? Normally you can apply a vinyl sealer (or shellac) and then be able to get away with applying a finish that is one degree away from the finish below - precat over a NC, or a conversion varnish over a precat. If the existing finish is more than a couple of years old my guess is that it is either a very low grade precat or an NC. Either one will probably give you problems if you recoat it with one of today’s high performance conversion varnishes. Instead I would use a vinyl and then a high quality precat to be safe. Use something stronger and be prepared to face the consequences. The combination of the vinyl with some of today’s precats can give terrific protection. I know that you are trying to do a favor for the customer - just remember that the road to hell is paved with good intentions. Do a test before you make a decision.
