I have just started spraying lacquer without using a sealer. I mixed 1 part thinner to 2 parts lacquer and got good results. I sanded to 220 on oak ply, then sprayed 4 coats of the mix, allowing only enough time to dry to touch in between. The gloss lacquer has a semi-gloss appearance (though this may be subjective), and the finish is smooth and even. I would say a success. Since it's my first time with lacquer, should I expect anything to go awry using this procedure?
From contributor D:
In fact, it's actually better NOT to use a sealer. The sealer is used only because it's easier to sand. Zinc stearate, the material with which the sealer is loaded so that it is easier to sand, softens the film and decreases the water resistance of the lacquer topcoats. It's used as a quicker way to get a smooth surface as it also fills the wood pores. If you don't mind the harder sanding and the quicker loading of your sandpaper, what you're doing is fine and in fact superior. Vinyl sealer, on the other hand, does present some advantages with respect to water resistance, but conventional stearated lacquer sanding sealer is a crutch, not a better way to do finishing.
Sealers are designed differently for a reason. Fast dry and a little harder film so when you sand, the wood fibers break off clean, resulting in a smoother base. Easy sanding is not a negative. They are also built to promote adhesion to wood, stains, etc.
Lacquer topcoats are slower drying and more flexible. Their sanding capabilities are marginal, especially on softer wood, poor sanding--poor quality.
Build coats with topcoat because of their flexibility. Sanding between coats is a plus.
Moisture resistance in pure lacquer is poor no matter how you spray it. Vinyl sealer helps, but in relative terms...not much.
If you want a beautiful furniture finish, use lacquer. If you want higher integrity films, you have to use high integrity coatings.
Laying down too much sealer is not good, but using the right amount can have many benefits. The drive for self-seal systems stems from the mid-sized shop that does not want two products in-house. All large production shops still use sealers along with the finer furniture companies. All these systems have their place. I don't feel that self-seal will ever take over.
Bob Niemeyer, forum technical advisor
Also, the pre-cat that I favor (Unicoat from MacLac - R. J. McGlennon in San Francisco) sands just as easily as most vinyl sealers. It seems to me that sealers are used exclusively to speed production (they dry faster and fill pores because of the stearate) but they do not improve the performance of the coating system. The exception to this is sealers used as barrier coats such as urethane for polyester or vinyl sealer for oil stains. These serve a distinct purpose as bridging films. But old-fashioned stearated lacquer sealer does not.
With a quality coating line, which nowadays everyone has available to them, sanding sealers are designed to do a job. Vinyl is used to bridge over the grain, helping seal the wood from moisture. Most quality sealers are designed to set up before sinking into the wood, keeping the coating on top of the surface, yet attaching to the wood at the same time. When you get into coatings other then NC, almost all of them make excellent self-sealers. Usually, they need to be thinned down a good deal, but they have good bridging properties and stay on the surface. Adhesion becomes an important factor and CV or pre-cats are much more sensitive, making it very common to use them as a self-sealer. Going back to the original person who was using NC, it sounds like he doesn't have a large volume, so using his NC topcoat isn't a problem. If he had a large volume, he would use less coating by putting a good sealer into his system.
The only time I would put shellac into the system is with pine, where I might want to seal the knots or with water base when using white stains on oak.
Some of the best sealers I've seen are the two part urethanes from Italy. They work great under CV or polyesters. But if you design a topcoat to be a sealer, you are going to have to give up some topcoat qualities and vise versa.
I agree with what Bob said about the self-sealing systems (which I will never understand because of the resin being the same less stearate and solvent line up changes) being more suited for smaller shops using only one product. I want to add that the sealer needs to be thinned more than the topcoat for good adhesion. You never want to replace excess amounts of sanding sealer for less topcoat and more fill. Sanding sealer, when used correctly, is part of the foundation of a good quality finish. In production, you cannot live without it. Sanding sealer is designed to raise any loose grain, penetrate the wood and sand easily for further coats to have a smooth surface to bond to.
Becker Acroma's product line is almost exclusively self-sealing. From their pre-catalysed Intro to their extremely high-solid conversion varnish Euroclear (56%), these products are meant to be used as sealer, as well as topcoat. Vinyl and sanding sealers are usually softer solids. Using a self-sealing product will give you a more protective finish. I have yet to come across a Becker product that is difficult to sand.
Comment from contributor A:
For a long time I did not use sanding sealers. Then one day, after spraying a 12' long double-sided book case for a school, I found the glue. A couple of nice 8-year-old size palm prints from my helper. I spent way too much time stripping and refinishing. Ever since then, I shoot a coat of sanding sealer first, just to be sure. I can't say the results are any different, but the job can be much easer.