A perfectly smooth finish is easier to achieve with some kinds of wood than others. For example, maple and birch have close grains that readily fill when you apply the finish. Other woods, notably oak and mahogany, have more open grains that aren't as easy to fill with finish, and many refinishers use paste fillers. Paste fillers aren't clear and must be color-matched to blend with the wood, but applying one presents no particular difficulties.
The Need for Paste Filler
Most lacquer and varnish products have enough solid material to fill small pores, and a paste filler isn't always necessary, especially when working with a close-grained wood. Finishers sometimes apply a base coat of sanding sealer and sand it flat before top-coating for the express purpose of filling the grain. This doesn't work with open-grain species like oak, however, because no finish can compensate for the height variations of the grain. Without a heavier material like a paste to fill depressions, visible contours on the surface are inevitable, no matter how many finish coats you apply.
Paste Filler Characteristics
Wood grain filler is similar to wood putty, but it is much thinner and unsuitable for repairing holes or cracks. The binder is usually varnish, and the bulk is provided by a finely ground material, such as calcium carbonate or silica. Paste fillers aren't clear, and most include a wood-tone pigment so they blend with the wood for which their use is intended. Their drying time depends on the binding material and can be from one to several hours. Once it dries, you can sand paste filler and coat it with any type of finish, but you can't change the color.
When to Apply Paste Filler
The best time to apply paste filler depends on whether or not you plan to stain the wood. Paste filler darkens the wood and acts as a stain when applied to bare wood. If you want to avoid either effect, apply a wash coat of finish, reduced with 50 percent thinner, before filling. Even though it raises the wood grain, you should avoid sanding until you spread the filler. If you plan to use an additional stain, you should apply it after filling. That way, you won't remove any stain when you sand the filler.
How to Spread Filler
Properly thinned paste filler, which should be the consistency of heavy cream, is too thin to apply with a putty knife. A better method is to spread it over the surface with an old paintbrush, then use a plastic paint scraper or a credit card to work it into the pores. When it turns dull, rubbing against the grain with a coarse rag is the best way to remove the excess. After it dries, sand the surface with 320-grit sandpaper, going with the grain. When you're done, shining a light on the surface will reveal any pores that need to be filled by a second application.
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