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Closed Pore Finishes: Don’t Forget About Grain Filler

1/20/2014 2:10:18 PM
Article by Bob Bein

Hello again, this time I would like to talk about an additional way to achieve a full, or “closed pore” finish. I have spoken in the past about using high solid, post catalyzed finishes such as catalyzed urethanes or polyesters. These products, when used correctly, can be applied using multiple coats without fear of cracking while filling the pores of even the most open pored woods. Unfortunately, those products are not suitable for all finishers and finishing situations.

For small and medium sizes projects, consider using a sometimes forgotten solution— grain filler. Grain filler has been used in the wood finishing process, in some form, for more than one hundred years. Today’s products are more user-friendly than formulations used in the past and can be applied in more of a production application. They fall into two basic categories: water-based and solvent-based. There are pluses and minuses to both, and both are applied very differently. Let’s start with solvent-based fillers. All of the major coatings companies and DIY manufacturers make solvent-based grain filler or paste wood filler as it is sometimes called. While there are some minor differences between brands they all share some common attributes. All are based in aliphatic solvents such as mineral spirits or naphtha. All have a thick consistency varying from the thickness of heavy cream to the consistency of spackle. All are made with a combination of the solvents mentioned above, binders and high solid grain fillers like silica, calcium carbonate, or barlite. They are all available in a neutral base that has no color. Some companies offer the filler in colors, while others offer tinting pastes to make your own color.

Application of solvent-based fillers can be done in two ways, depending on the effect you are looking for. You can apply the grain filler directly to the wood and stain the wood while filling the pores or you can apply a stain, then a wash coat, and then apply the filler over the wash coat. An important part of the color matching process involves pore control. By this I mean that matching a stain has two components: the color of the flake, or background, and the color of the pores. When separation of background and pore color are not important, it is possible to use the first method, staining and pore filling at the same time with a grain filler. First, sand the raw wood with 150 grit sand paper. Start with grain filler that is close to the final color or add tinting pastes as needed to achieve the final color. Make sure the consistency of the grain filler is similar to heavy cream or a milk shake. If the filler is thicker than this add mineral spirits until it is the proper consistency. For faster drying you can add naphtha in lieu of the mineral spirits. Apply the grain filler to the wood using a brush or pour a small amount onto the surface to be filled and spread with a plastic squeegee. Vertical surfaces can be done with care, but grain filler is best applied to horizontal surfaces whenever possible. You need to push the filler down into the pores of the wood. With a brush you want to use a circular motion. With a squeegee you want to go diagonally across the grain. Use the brush to remove the excess filler until a thin coat is sitting on the surface. Let the filler start to dry, to the point that you can see a lighter color to the filler at the high points of the film. If the filler is too wet you will remove the filler from the pores, too dry and it will be as hard as a rock to remove. At this point, take a piece of burlap cloth or scotchbrite pad and remove the excess filler. When you are done it should feel dry. You can wipe the surface with a rag that is DAMPENED, NOT SATURATED with Naphtha or mineral spirits. This will soften and remove any residue that is left on the surface of the wood. Let the filler dry overnight. Sand the surface lightly the next day with 220 or 280 grit sand paper. For exceptionally deep-grained woods, it may be necessary to apply a second coat of filler. Apply a light coat of sealer before applying full wet coats.

The second method used to fill the grain involves using a wash coat. This method will give you better control over the color of the pores. This is because the staining and pore filling will be done separately with a thin layer of sealer in between. First, after properly sanding the wood, stain the wood with the stain system of your choice. When the stain is dry apply a wash coat of sealer, preferably nitrocellulose or pre-catalyzed sealer. This wash coat mixture should be 50% sealer and 50% reducer. For shellacaholics use a ½ pound or ¾ pound cut shellac. Spray one even coat. Let it dry and sand lightly with 280 or 320 grit sand paper. Make the grain filler a little thinner than when you are applying it directly to the wood. This ensures that it gets completely into the grain of the wood. Apply with a brush or squeegee in the same manner as described above, making sure to go against the grain or diagonal to the grain. When it starts to look hazy, it is dry enough to wipe. Use a cotton cloth and wipe it completely dry, without using too much pressure. Again, a damp cloth with naphtha can be used to remove the residue. Please be aware that the filler will affect the final color of the project. This should be taken into consideration during the color development stage when making the sample for submittal.

Water base wood filler is a recent addition to the market and less popular than its solvent-based counterpart. It does, however, have several advantages over solvent-based filler and is the filler of choice whenever I am doing a restoration project. It dries much faster than solvent-based filler and shrinks less after it has dried. It has one caveat, however. It can only be applied to raw wood. In most cases, it is applied in its clear state. To apply it correctly, it is usually at the correct viscosity right out of the can. Pour a small amount onto the part to be filled. Using a plastic squeegee, move the filler across the entire surface. Keep the angle of the squeegee low and attempt to push the filler into the pores of the wood. Move the squeegee diagonally across the grain. Remove most of the excess but leave a thin film on the surface of the wood so it can shrink back into the pores while it dries; just don’t leave the film too thick. The filler will be dry enough to sand in two to three hours. Sand the filler down to the surface of the wood starting with 180 grit sand paper and go down to 220 or 280 grit. You can now apply water-based wiping stain on top of the filled surface. This will color the wood but it will not give you strong contrast in the grain. If you need stronger contrast in the grain, then you can tint the wood filler to the correct color in the grain of the wood using water-based tinting pastes. Sand the surface smooth and apply a wiping stain. This method will leave a darker, more pronounced grain. For water-based finishing you are ready to apply the first coat of sealer. For solvent-based finishing, wait overnight before applying the sealer. For semi-open pore finish, use the techniques above, but add more reducer to the filler. This is true with solvent-based or water-based fillers. Revisit these time-honored methods and you will not be disappointed. Until next time, keep finishing.

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