A wood grain filler is a paste or liquid used for filling in the holes on open-grained wood before layers of traditional wood finish are applied. It smoothes the surface and provides a good base for wood finishes.
Softwood does not require wood grain filler. Hardwoods can be open or closed grained and this distinction will predict whether or not it will need wood grain filler in order to produce a desired finish to the wood.
Open Grained Woods versus Closed Grain Woods
Wood has a grain that is similar to pores in skin. These pores vary in size depending on the type of wood. Oak, cherry, walnut, ash, chestnut, elm, hickory, mahogany and others are all considered large pored or open grained wood types. These holes in the surface give the wood an unsmooth texture. When a stain is applied, these pores cause the stain to pool in them and will absorb more stain than the rest of the wood. This results in the pores becoming darker than the surrounding area. So open grained woods will benefit from a paste type wood filler.
Maple, beech, aspen, cottonwood, sycamore, magnolia, yellow-poplar, alder and others are all considered closed grain wood types. The pores are very small and the wood is smooth. Applying stain to any of these closed grain woods will result in a consistently colored wood surface with a naturally smooth feel, without need for a filler.
Components of Wood Grain Filler
Wood grain filler can be oil-based or water-based. Oil-based fillers take longer to dry than water-based. Either way it consists of three components. They are referred to as the binder, the bulking agent and a solvent. The binder is a wood finish. In the case of an oil-based filler the binder is usually made of a blend of oil and varnish.
In the case of water-based filler, the binder is usually acrylic or urethane. The type of binder determines the type of solvent used. For oil-based filler it is usually mineral spirits. For water-based filler the solvent is water.
Oil-based and water-based fillers both contain silica in the bulking agent because it resists swelling and shrinking even with temperature and humidity changes.
Wood grain fillers come in different colors that can be matched to the natural color of the wood or contrasted, depending on what final appearance it is that you desire. A light colored oil-based filler can be mixed with other colors of oil-based filler to achieve whatever color you like.
In addition, you can add universal colorants to an oil-based filler to create a custom color. For water-based fillers you can add a dry powder pigment to get the color that you desire.
Application of Wood Grain Filler
The process of filling the grain usually starts with sanding the entire surface of the wood. This is to smooth away any small imperfections that may become more visible with the application of the filler. Then you begin to apply the filler.
For paste wood grain filler, the consistency should be similar to thick cream. Thin as required, following the filler manufacturers recommendations; turpentine is typically called for. Stir the filler thoroughly before application, and often during use so that it does not harden.
Application can be done with a brush, a cloth or applied with a scraper. You should apply the filler in the directions of both with and against the grain. Use a circular motion to make sure that each pore is completely filled up.
At first, the filler will have a glossy appearance. In a few minutes it will begin to look dull. As it dulls it is beginning to harden. You will need to remove any excess filler before it dries. Once dry it becomes difficult to remove. Work on a small area at a time so that you are able to keep up with removing excess filler before it dries.
Removing excess filler can be done in several ways. You can remove it by rubbing it off with an old towel, or burlap. A plastic piece of mesh will also work. Scrub across the grain until the filler is completely removed from the surface of the wood.
For larger amounts of excess filler, you can use a plastic scraper or a stiff putty knife. Move the tool along the direction of the grain, holding it at an angle, cleaning filler from the edge of the tool with each pass. The less filler that you remove before it dries, the harder it will be to sand off once hardened.
Depending on the size of the grain and type of wood, you may need to put more than one coat of filler on it. The goal is to have all the pores level with the surface of the wood. When all the pores are uniformly filled to the surface, all the excess is removed and the filler is dried, it is time to sand again. Once the entire surface has been sanded smooth wipe it off with a cloth to remove any sawdust or residue. Follow with the final finish of your choice.
If you are inexperienced with the type of wood grain, particular type of filler you have chosen to use or any other aspect of the process try it out on a scrap of wood. It is much better to learn which application method or excess removal style works best while testing it on a scrap rather than the actual wood piece. You may also want to sand the practice piece as well so that you can test to see how the final clear finish will appear. In fact, test the entire sequence of staining, filling and varnishing so you will know the end effect. It is always better to be safe than sorry.