Wood finishing is both art and science, and anyone who has ever stained an unfinished pine bookshelf or stripped and refinished an antique maple dresser has learned that the final result can be different than initially planned. Although they serve much different purposes, sanding sealers and prestain conditioners can help make wood finishing much more predictable.
The Structure of Wood
Wood is an absorbent material, and softwoods such as pine and cedar are much more spongelike under the surface than hardwoods such as maple and mahogany. Softwoods are also lighter in color and display more variation in their grain patterns. As a result, penetrating stains may settle unevenly in the wood, leading to a blotchy-colored appearance. Blotching appears uglier in lighter-colored softwoods than burled or darker softwoods, but it is not a serious problem in hardwoods.
How Prestain Conditioner Works
A prestain wood conditioner is actually a very diluted version of a varnish or sealant, and it is designed to penetrate beneath the wood's surface, where it performs as barrier to deeper absorption of colored wood stains. Oil-based prestain treatments take up to a day to dry. Water-based prestains dry in less than two hours but require light sanding before a stain can be applied. While a prestain conditioner can prevent blotching, it may make it necessary to apply two coats of stain to achieve the color you want.
How Sanding Sealers Work
Sanding sealers are essentially a clear wood primer that can precede the application of a clear wood finish such as varnish, shellac or polyurethane. The sanding sealer provides a thin film that prevents stain bleed-through into the finish coat, and also prevents wood finishes from penetrating unevenly into the wood. Just as important, it prevents the finish coat from interacting with residual furniture polish or wax, which can lead an unsightly "fish-eye" blemish to appear in the finish after application.
Getting the Order Right
The prestain conditioner should only be applied before the stain. A sanding sealer must be applied after the stain but before the finish coat because otherwise it would prevent the stain from penetrating. A water-based sanding sealer can serve as an intermediary coat between, for instance, an oil-based penetrating stain and a water-based polyurethane. As the name suggests, sanding sealers can be cleanly sanded to smooth wood surfaces. That can be important when a water-based prestain and stain raises the wood grain, leading to an uneven surface.