The traditional sealer for shellac, lacquer, and natural varnish finishes is thinned white shellac. This basic sealer is simply a mixture of 1 part white shellac (4-pound cut) and 3 to 4 parts denatured alcohol. Shellac is suitable for most refinishing jobs, but it cannot be used with polyurethane varnish or with water or NGR (non-grain-raising) stains.
Where shellac cannot be used, the easiest sealer is a commercial sanding sealer. Sanding sealer dries quickly and provides a very good sanding base; it can be used with varnish, shellac, or lacquer. If you plan to finish the piece with polyurethane varnish, read the label carefully; sanding sealer may not be compatible with polyurethane. Sealing is not necessary before finishing with a penetrating resin sealer.
Under natural varnish or lacquer finishes, some professionals prefer to seal the wood with a thinned mixture of the same finish. To make a natural varnish sealer, thin the varnish with turpentine or mineral spirits to make a 50-50 mixture. To make lacquer sealer, mix lacquer and lacquer thinner in equal parts. These sealers cannot be used with shellac or with polyurethane varnish.
Polyurethane varnish demands special treatment. Read the labels carefully when you buy. Some polyurethanes can be thinned with a specific thinner; with these varnishes, the manufacturer may recommend thin varnish coats as sealers. Some polyurethanes do not require sealers. If you must seal stain or filler before polyurethane is applied, make sure the sealer is compatible with the varnish. Otherwise, use a penetrating resin sealer. This finishes the wood completely, but you can apply polyurethane over it if you want a smoother finish.