Shellac as a Sealer under Lacquer
From Paul Snyder, forum technical advisor:
The question that comes to my mind is why do you plan to use shellac as a sealer?
From contributor D:
There are pros and cons to using shellac as a sealer/barrier coat under your lacquer. There are many pros. It increases depth and forms a barrier coat to seal in possible contaminants, promotes adhesion of the topcoat to the substrate, and etc.
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.
From contributor M:
You might want to check to see if you NC is self-sealing. Usually shellac is used as a barrier between dissimilar coatings. Shellac is a good sealer, but I am wondering why you would want to use it under lacquer. You make reference to fish-eye. This is usually caused by some sort of contaminate on the surface prior to spraying. Silicone is the usual offender. To eliminate this, I usually wipe the surface with Naptha or Lacquer thinner.
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.
From contributor A:
Shellac certainly has its place in finishing, it has many excellent qualities, including being food safe. I personally prefer using it for doing furniture restorations. Another feature that's worth mentioning about Shellac is that its rubs out and polishes up the sheen of almost any furniture.
From contributor B:
The sanding sealer that Zinnser puts out as a 1# cut is de-waxed. I think that most of the premixed 2 and 3 cuts are not de-waxed, and this can cause bonding issues with subsequent finishes. I like to use the number 1 cut and spray it as a sealer coat on nearly everything that I finish. I have never tried using shellac that has not been de-waxed, unless I am using it as the finished coat.
From the original questioner:
What I have in mind is this: I often find fish-eyes on the tops of tables I refinish, or on drawer fronts around the knobs, and rarely all over. My habit is to strip, followed by a good washing with lacquer thinner to eliminate fish-eyes. I have also tried acetone and another solvent I don't remember. So far, my best plan has been to mist on about six coats of lacquer sealer, followed by a heavier, wet coat, then sponge pad sand it flat, then begin with the standard lacquer product. I want to find a better method than that of misting so many seal coats, if possible.
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?
From Paul Snyder, forum technical advisor:
To the original questioner: Avoiding contamination (fish-eye) is a good reason to start with a shellac sealer. With nitrocellulose lacquer, it doesn't matter if the shellac is de-waxed or not. Zinser does make a product called Seal Coat that is a de-waxed shellac mixed to a 2# cut ready for use as a sealer. Don't over-do the shellac when you're top coating with lacquer; you want to keep the shellac thin. One good coat is all you need. Too thick and it can lead to pre-mature cracking/crazing down the road.
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.
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