Will Lacquer Re-Dissolve Shellac?
Most top-coats include some sort of solvent that could melt into shellac. Here's advice on prep sanding and other issues relating to shellac as a bond coat. May 18, 2010
Question
I am using this sealer as a bond coat on 40 finished doors that I have to tone a bit darker. Will lacquer re-melt this shellac sealer or it will be better to scuff before toning?
Forum Responses
(Finishing Forum)
From contributor K:
As long as there is no dirt in the SealCoat you shouldn’t need to sand it.
From contributor H:
My experience is that lacquer likes a slightly scratched surface. It gives it some teeth to bond with. I wouldn't risk an adhesion issue. A quick rubout with a sanding sponge takes no time.
From contributor Y:
Since you’re stating that you’re using this strictly as a bonding coat it is safe to sand it. Though you will have melting of the finish into one homogeneous coating if you’re using a straight nitrocellulose type lacquer. The pre-cats and post-cats - not so much. This is because the ratio of solvent and types differ depending on the mfg. So if you use a belt and suspenders approach you'll be safe.
From contributor M:
Since shellac is not catalyzed, if you spray lacquer over it within two hours you don't think it will re-melt. I just like to know my options. I agree with the scuffing. I always do it.
From contributor Y:
No, it will re-melt without sanding , depending on the lacquer used - some more than others. As long as the coating has some type of solvent (that incudes waterbourne that has butyl cellosolve in it) that will melt or re-liquify the shellac to whatever degree. Allow bonding into one homogenous film to occur, then you’re ok. As long as the finish underneath is sound (no cracks/good adhesion/etc.) and they are chemically clean (no wax or other cleaning contaminates) the job should go smooth, that said, some cracking may not be readily visible by eye and you might want to check for cracks under magnification on a door or two just to make sure this will not become a future problem.
From contributor L:
I usually use water base products unless there is a need to hold on to natural reds, cherry, mahogany, etc. Then, I have used seal coat under them as a sealer. I was asked to do a refinish job and planned on using water based toners with metal dyes. The backs of the doors came out great, but the fronts were an issue. Apparently, someone used a cleaning, furniture polish or wax product that really wrecked havoc with the water based product. I sanded, cleaned and used the seal coat as a sealer, but the contaminant floated thru. I tried using the seal coat mixed as a toner, over the sealer, and it worked great. The metal dyes mixed ok with the seal coat. The water based top coat then had no problems.
