Cracking of Lacquer over Shellac

Multiple shellac coats (sanded between coats) lead to trouble with later lacquer coats in this example. March 29, 2008

Finished a cherry cabinet first week of December and it looked great. Let it stand 10 days to harden, and when I went to buff, the surface was cracked in several corners (on a diagonal, not joint failure).

Finished as follows:
1. Olympic wipe stain
2. 2 coats Seal Coat, sand between coats
3. 2 coats Valspar 1420 clear lacquer sealer/undercoat. sand after 220 grit
3. 2 coats Valspar 1429 gloss lacquer

After seeing the cracking, sprayed with 50/50 mix of lacquer thinner and retarder, and the cracks melted in a bit, but were still there. Then I sanded down heavily (broke through in a few places), resprayed 1 coat of gloss topcoat. Finish looked good, no cracks. Reassembled the cabinet on 12/22. Looked good still on 12/27 when I left on vacation. Upon return on 12/30, the job was full of cracks in the same and new areas.

Called the paint dealer, which was useless. Finally got Valspar rep on phone and he said it was probably a chemical reaction between the Sealcoat and the lacquer... or paint on too thick. According to the dealer, this is NC lacquer and can be built up. According to Valspar, there is a 4 dry mil max, but calculating the solids percentage, it's marginal if it is over 4 mils. 6 coats of a 20% solids paint, minus sanding loss, I figured was okay.

I am now stripping and redoing, which the Valspar rep said to do, and not to use the SealCoat. As he put it, "they have no idea what is in the Sealcoat." Hard for me to understand that. Additionally, the Sealcoat can says "guaranteed to work under any clear topcoat."

Does anyone know what could have happened? I'd like to avoid a third episode with this cabinet. Previously I was using Deft lacquer and never had a problem with application or customer satisfaction. My own kitchen is almost 25 years old with Deft and still looks good, but I am having a hard time getting Deft anymore due to the AQMD restrictions.

Forum Responses
(Finishing Forum)

From contributor C:
Switch to Mohawk lacquers. Use their vinyl sealer for first coat and their high solids lacquer for all other coats. I have been doing piano finishes for 40+ years with no cracking. I am not familiar with Valspar, but it sounds like a sealer problem for sure - not a topcoat problem.

From contributor W:
Sealcoat is a shellac product, not a lacquer one, if it's the same Sealcoat that is put out by Zinnser.

From contributor C:
Yes - shellac pure dewaxed can be used under virtually any solvent finish and many waters also, but you cannot build with it, nor should you sand it directly, for if you do, the barrier properties of the product will be null and void. I always apply a lacquer sealer or vinyl sealer over the (1) coat of 2 lb and lightly sand it instead, being careful not to sand too much - just to remove raised wood fiber. Then if necessary another coat of 50/50 vinyl or lacquer sealer can be applied and more careful sanding done. It is always best to not apply more than one coat of sealer unthinned, or two if thinned 1 to 1 with lacquer thinner. Also try to use a nitro lacquer designed for more rigorous work - a bar top type with a coconut oil alkyd resin will give better protection against water and such without having any reactive problems if heavy build is needed.

From contributor D:
You whip out the shellac (Seal-Coat) when you've got a compatibility issue or you need a decent color under waterborne finishes that without it look dead. Use lacquer sanding sealer and lacquer and forget about the shellac. What purpose does it serve? Valspar makes great stuff and it's used by the tanker car in high volume operations. These guys don't make crap paint.

From contributor B:
If there are cracks it is because the film thickness was excessive. The only resolution is to strip it down to the stain and re-spray. I would leave out the Seal Coat, not that it was hurting anything, but because it is not giving you any benefit that the lacquer can not provide. Additionally, no manufacturer of coatings will accept the use of another manufacturer's product in conjunction with their products (too much liability).

From contributor C:
True nitro lacquers are an evaporative finish, like other evaporative finishes such as acrylics. Nitro is able to be built to more thickness than the newer catalytic coatings of nitro or alkyd or other. You must remember it was not that long ago that all cars were either finished with nitro or acrylic finishes. Many of the fancy finishes such as metal flakes or metallic back then needed 30 or more coats of build to look deep and clear. You cannot do this type of work with modern coatings now flooding the marketplace. The other good thing about them is that they are easily repairable as compared to new coatings. Their drawback? High VOCs, but I could care less. I do not use in volume anyway, and if I did, it would still be a better choice than any reactive finish out there today.

From contributor B:
I still think the shellac is the culprit. Years ago I used shellac to get a crackle effect.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor E:
I had a nightmare of a time when I applied Zinsser sealer to a marquetry panel, sanded it for adhesion purposes, and then applied a West System epoxy over that. Everything was fine until the cat lacquer was applied and then after a week, the finish lifted at the layer of the sealer and the epoxy. This happened three times all while I sanded the sealer further down to where I thought i would get to wood. It finally worked and has not been a problem since. I came to find out that the sealer was several years old and that might have been the problem. Shellac sealer gives an incredible coat that is fantastic for the appearance of good veneer joinery. I wish it worked great with other finishes.