Barrier Coat Between Polyurethane and Lacquer

      De-waxed shellac should work well to protect an existing waterborne poly from a new coat of pre-catalyzed lacquer. May 7, 2007

Question
Does anyone out there know if pre-cat lacquer can be sprayed on top of acrylic polyurethane?

Forum Responses
(Finishing Forum)
From contributor P:
You can usually use shellac as a buffer coat between any two finishes. I've sprayed lacquer over shellac without difficulty. I think your problem is going to be getting the shellac to adhere properly to the poly. Try a little spot and then try the masking tape test to see how your adhesion sticks.



From contributor W:
Scuff sand first before shellac coat.


From the original questioner:
Thanks for the input. I have never used shellac before but have read about people using it. What do you think about a vinyl sealer?


From contributor B:
Your vinyl sealer is still a lacquer base and will make a mess of the poly if you don't use shellac first. You probably don't need to use the vinyl sealer after shellac over poly. Just topcoat with your lacquer.


From the original questioner:
Thanks. Looks like you just saved me from making a mess. I was going to test it but now I'll just use the shellac.


From contributor D:
If it's 2K acrylic urethane, lacquer thinner will not even touch it, so no mess will be made whatsoever. Scuff and blow down the lacquer and you'll be fine. Acrylic urethane is automotive clear coat, and once hardened, it's impervious to solvent attack except for the most serious of chemical strippers (methylene chloride).

If on the other hand it's that lame Minwax waterborne crap you buy at the Home Depot, the shellac recommendation is mandatory.



From contributor Y:
Check with your paint supplier - he should have a barrier coat you can spray. Chemcraft has a product called Isolante 401-033. ICA also has a few different versions of Isolante - IS 203, IS 207 to name a couple. We recommend these products when there is a metal or plastic inlay for better adhesion.


From the original questioner:
I guess I should have been more specific. I was asked to darken the finish on some custom speakers, $$$. All I was told is that their finish was water based “Clear Cote.” I searched the web and came up with "Clear Cote Clear Acrylic Polyurethane - Satin 58009" from California Paints. Their sheet says it is an acrylic polyurethane and it thins with water. They also say when used over previously finished wood, if there is wax, shellac or a stearate finish, they have to be completely removed. The stock products I have are all Mohawk Brand. I had planned on tinting the E-Z Vinyl Sealer with the Ultra Penetrating Dye Stain and finishing up with DuraCoat Pre Catalyzed Lacquer, tinted with the Ultra Penetrating Dye Stain if needed.


From contributor E:
I would use Zinsser's seal-coat. It's a de-waxed shellac which will be perfect for your need.

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