Like good sanding, careful sealing can make all the difference to your results in refinishing furniture. Sealer coats are used between finishing steps to ensure even penetration of stains and finishes, to prevent bleeding of stains and fillers, to form a good base for the finish, and to make the finished surface smoother. In this article, we'll discuss some effective ways to seal wooden furniture.
Choosing a Sealer
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.
Because sealers should not interfere with other finishing steps, they must be applied in thin coats. Before applying the sealer, make sure the wood is clean; remove any dust with a tack cloth. Apply the sealer with a clean brush, flowing it on evenly and quickly along the grain of the wood. Make sure all surfaces are evenly covered, and pay particular attention to any end grain. End grain that isn't properly sealed will absorb stains and finishes more deeply than the rest of the wood in a piece.
Let the sealer dry completely: about two hours for thinned white shellac, about one hour for commercial sanding sealer. Then sand the surface very lightly with fine-grit sandpaper, grade 7/0. The wood must be very smooth, but the sanding shouldn't penetrate the sealer. Remove all sanding debris with a tack cloth.
If you're applying a finish directly over sanded wood, more than one coat of sealer may be necessary to close the wood's pores completely. In this case, let the first coat of sealer dry completely before applying another coat. Very porous woods may require several coats of sealer.
If you're expecting your refinishing work to last, and you're expecting a high-quality return for your investment, proper sealing can get you what you are looking for.
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