I am looking to use shellac as a sealer under nitrocellulose lacquer. I imagine the fish-eye problem could be reduced, but is this a good idea? Would shellac as a sealer compromise the integrity of the lacquer on top? If that is ok, what cut of shellac is recommended? Also, what is the drying time for shellac, and are there any known pitfalls? Any help is appreciated.
From contributor R:
What type of projects do you plan on finishing with shellac as a base coat?
The cons are that you must either mix your own, or buy a new can of Zinsser's Sealcoat. Always look for the date on the can so that you know you are buying fresh Sealcoat (under a year despite the claims of a three year shelf life). Always test the shellac by dripping some onto a piece of glass and seeing how long it takes for that drip to dry. The deader the shellac the longer it takes to dry.
I think that it is a good move. The dry time for your Sealcoat is about 20 to 30 minutes, unless you dont mind your sandpaper corning and clogging. I think that you touched on the most important feature of a shellac sealer, and that is its ability to act as a barrier coat for subsequent topcoats. I mentioned Sealcoat because this is a great product. I like that it is premixed and I also like the thin nature of the one pound cut.
I sum it all up as good insurance. The only real downside I see is that it takes a little longer for the stuff to dry compared to regular lacquer. And if you can dedicate a gun just for your shellac, then you can greatly minimize some downtime for changing and cleaning your gun as you switch out materials to be sprayed.
I do have a customer who specifies shellac as the first step on the schedule. While I consider myself to be an experienced finisher, I have had problems spraying shellac. For convenience, I have been using Bulls eye amber, in a 3# cut. I make my first coat a 2# cut. It does better settling in the pores. Then I can use the 3# on successive passes. I have found that I can get one wet coat on at a time and this is one pass. When I hit it a second time, there appears to be too much material and it does not dry properly, resulting in orange peel.
I also have had some adhesion problems with the lacquer over the shellac. The thinner in NC is not as hot as the alcohol in shellac. It will not bite, so a mechanical bond is necessary. When I have sanded with greater than 320, like with a used sanding sponge, I have been able to peel the lacquer off (although it does take some effort). So just make sure that you don't go too fine.
I have also had problems spraying shellac on porous woods. I had to put a third coat on and set it outside so the heat (80 degrees) and wind would help it dry. I went back an hour later and I had bubbles all over the surface. I guess that the air in the pores heated up, expanded, and needed some place to go.
I recently sprayed shellac for the first time - onto a surface that lacquer, poly, and varnish would not stick to. Since shellac was there to begin, this worked well, but what if I don't want a purely shellac finish? Could I use shellac more often, covering fish eyes, and have lacquer on top? The shellac I sprayed was Zinser 3# white, cut in half with the appropriate alcohol. Should I use wax? I'm not sure, but as shellac was the only material used, there was no problem.
Also, about the spray gun cleaning - I use only one gun, spraying lacquer thinner through it after every use. Would there be a problem using the same gun with de-waxed shellac and lacquer?
As far as your spray gun goes, denatured alcohol cleans up shellac better than lacquer thinner. Lacquer thinner kicks the shellac out of solution and you get little chunks that have to be manually removed (strip the gun and run a brush through the fluid passages). When you're done spraying shellac, run some alcohol through the gun and then run some lacquer thinner through it. That'll get it ready to switch over to lacquer.
Comment from contributor J:
I have been using shellac as a sealer under lacquer for years. It seals contaminants on refinished wood very well. The drawback is that if the shellac is not completely thick or not dried completely (should dry a few days) it will bubble up under your lacquer finish. Then you need to start all over.