Filling Wood Grain
DIY Furniture Repair
Amateur refinisher's often overlook an important step in the finishing process: filling wood grain. Though not absolutely necessary, this step can greatly improve the appearance of the final finish.
Some open-grain furniture woods that commonly get a filler treatment include mahogany, walnut, oak, ash, rosewood, chestnut, butternut, and elm.
If you're finishing with a film finish like lacquer, shellac, or varnish (polyurethane), fill any of these woods before you apply the finish. Without filler, the wood will generally appear grainy and porous. The only time filling wood grain is not recommended is when you're finishing open-grain woods with oil or wax.
Paste wood filler
Wood, or grain, filler is a paste-like substance that can be applied to open-grain woods after sanding and before finishing. Because filler is not meant to stay on the wood's surface but only in the open grain crevices, you must remove excess filler before it hardens.
After the material dries, sand the surface smooth with fine sandpaper. This process provides a smooth, level, non-porous surface for finish application.
Oil or water-based?
Wood filler is available in both oil-based and water-based forms, and either type will do the job. There are a few differences between the two, primarily in the consistency of the filler and the amount of drying time needed before you can apply the finish.
Oil-based filler is thicker than water-based filler and, because of its oil content, requires at least overnight (and possibly longer) to dry.
Water-based filler dries more quickly, and that means you have less working time with the material: it can be finish-coated in as little as two hours.
Another small but potentially important difference is that oil-based filler may not be compatible with water-based finishes, while water-based filler, once dry, is compatible with both water and solvent-based finishes. Whichever filler you decide to use, be sure to read and follow label directions closely.
Filler color
Wood filler is available in a variety of colors, including natural. While wood filler is not meant to add a lot of color to wood, it will have some effect, primarily in the open grain of the wood since this is where most of the filler stays.
If you want the grain to look darker, use a dark-colored filler; if you're trying to de emphasize the wood grain, making it more subtle, use a lighter-colored filler. Most woods will look better when filled with a filler that is a little darker than the wood itself, as this will enhance the grain rather than blend it in with the surrounding wood color.
Staining before filling
After stripping and sanding an open-grain wood, the next step is usually filling wood grain. There are some exceptions, however. When you're going to use a dye stain to color the wood, for example, you might want to stain the wood before filling it. The reason for staining before filling wood grain in this case is that dye stains must penetrate the wood to color it, and wood filler can hinder this penetration.
This is not true when using pigmented stains, because they add color primarily on top of the wood. One drawback to staining the wood before filling it is that you can cut through the stain color when you sand the wood filler, the last step in the filling wood grain process. Such cut-through areas can be difficult to blend in with the surrounding color.
Using a water-soluble aniline dye can diminish this difficulty. Use the dye to stain the wood before filling it. You can then use the dye to restrain the cut-through surface after you've completed the filling wood grain and sanding steps. A water-soluble dye stain will more easily blend the cut-through areas with the surrounding color.
Sealing before filling
Sealing the wood before filling wood grain may be necessary if you want to use a dark-colored filler but you don't want to darken the overall wood color. Sealing the wood first prevents the dark filler from significantly darkening the wood surface while still allowing it to fill and add prominence to the wood grain. If you decide to seal the wood surface before filling it, use a thinned coat of lacquer for spray sealing and a thinned coat of shellac for brush sealing. After the sealer coat dries, sand the surface lightly using extra-fine sandpaper (280-grit).
After tacking the surface to remove sanding dust, proceed with the filling wood grain process:
Tools and materials you need:
• wood filler
• latex gloves
• paintbrush
• coarse rags
• 220-grit stearated sandpaper
• rubber squeegee or plastic scraper
• #0000 steel wool or nylon abrasive pad
• Because wood filler is so thick, you must thin it down to a brushable consistency before applying it to wood. Use mineral spirits to thin oil-based filler (or naphtha if you want a solvent that's a little faster drying) and water to thin water-based filler.
1) Apply filler to the wood
Mix filler (enough to do the job) in a separate container, and be sure to mix the thick pigment well into the solution so it has a thin, creamy consistency. If you're changing the filler color, add the necessary tinting color. Put on a pair of vinyl or latex dishwashing gloves, and use a regular paintbrush (natural bristle for oil-based or synthetic for water-based) to apply the filler to the wood. It's not important how you brush on the filler as long as you cover the entire wood surface, including cracks and crevices. Apply the filler to one small section of the piece at a time so you'll have time to work it into the wood before it dries.
2) Remove excess filler
As soon as the filler begins to dry (immediately with water-based and after a few minutes with oil-based) use a coarse rag such as burlap to begin removing the excess. Wipe across the grain as much as possible so you don't pull the filler out of the grain pores. You can also use a rubber squeegee or a plastic scraper with a broad blade to force the filler into the wood pores while at the same time scraping off the excess. If you use a scraper, be careful not to scratch the wood.
3) Pack the filler
After you've removed the majority of the excess filler from the surface, use the palms of your gloved hands to pack the filler down into the wood grain, rubbing in a circular or cross-grain motion. As you do this, the friction from your hands will cause the filler to dry more quickly, so you should go immediately to the next step.
4) Wipe across the grain
Using a clean rag, wipe over the wood surface to remove all excess filler. Wipe across the grain as much as possible to allow the filler to remain in the pores. Also at this point, clean excess filler from any corners, cracks, or crevices. You can use the point of a sharpened dowel to get at the filler, or use a screwdriver tip with a rag pulled over it. Be sure to get the wood surface as clean as possible without removing the filler from the wood grain.
5) Sand the surface
After the filler is completely dry, use 220-grit stearated paper to sand all flat wood surfaces. Sand in the direction of the grain. It's not necessary to sand heavily, only enough to smooth the surface. Rub out all turned or irregular areas with nylon abrasive pads or #0000 steel wool.
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Source: B.Hingley
