If you've ever finished or refinished a porous wood, such as pine, you've probably found the reason why professional refinishers recommend using wood conditioner. When you apply a stain to a porous wood, the stain soaks in unevenly and the final result is blotchy. A pre-coat of wood conditioner, which consists of a low concentration of a shellac, varnish, lacquer or wax in a solvent, prevents blotchiness by sealing the grain.
Commercial and Homemade Conditioners
Commercial wood conditioning products often consist of 1 part varnish thinned with 2 parts mineral spirits. Some professional refinishers make their own wood conditioner by dissolving a small amount of the clear finish they plan to use as a topcoat, such as lacquer, shellac or varnish, in an appropriate solvent. You can even buy water-based wood conditioners, which contain small concentrations of water-soluble acrylic solids.
Compensating for Uneven Grain
All wood conditioners -- no matter their composition -- have the same purpose: They seal the pores of the wood and prevent deep penetration of the stain. The result is a more uniform coloring when you apply a liquid stain. The benefits of wood conditioners are obvious with knotty pine, which is a soft wood with an irregular grain structure. When you stain this wood without conditioning it, the areas around the knots appear darker than the rest of the wood. The overall effect is chaotic, unsightly and difficult to correct. You get similar results when staining unconditioned birch and many other species.
Working With Wood Conditioner
If you use a commercial conditioner, the instructions on the container direct you to apply the product generously on clean, bare wood with a paintbrush or rag, to wipe off the excess within 15 minutes and to apply stain within two hours. Refinishing expert Bob Flexner claims to get the best results after waiting overnight. If you make your own conditioner from shellac or lacquer, it's best to give the sealer time to dry and cure, which could take from one to two hours. Complete all sanding before you apply conditioner, although you may want to scuff with 220-grit paper just before staining to knock down the grain raised by the conditioner -- especially if you use a water-based product.