Filling Wood Grain for Perfect Finishing

McKinney, Texas, woodworker Jim Kull, has been restoring furniture, building antique replications, and doing custom work full-time since he took early retirement over a decade ago. "It's something that I always wanted to do, but I wasn't expecting to set up shop until I was 62," Jim offers. "Today, I love restoration, and that includes wood finishing. So, when we wanted to get filled in on filling wood grain, we turned to Jim.

"If you want a glasslike finish, such as on a desk or a tabletop, you have to fill the grain-actually the wood pores in the grain," says Jim. "Oak is the best example. You can see an oak piece with either filled grain that has a smooth, reflective finish or with unfilled grain and a semi-rough look. The difference in texture is like glass versus fabric."

Yet, as with most anything, there's more to filling than smoothness, according to Jim. "Filling is more a matter of preference for the look you want or an effect that you are trying to achieve. Because in addition to finish texture, you can either accent the grain or subdue it-making it striking or bland."

There are essentially four ways to fill the pores of a coarse-grained wood like oak, walnut, and mahogany. And from Jim's experience, none of them prove easy.

"You can fill the pores with your finish material, whether it's varnish, lacquer, or water-base," he explains. "It just takes repeated coats with sanding in between. This won't accentuate the grain any more than it is naturally. Secondly, you can use a prepared water-based filler right out of the can. It takes stain, so you can highlight the pores, but it also has distinct disadvantages-like drying too quickly-that make it difficult to use. I don't bother with it."

That leaves two options: an oil slurry and commercial oil-based paste filler. Jim prefers the oil slurry, so we asked him to go through that process first.

To create a slurry to fill the grain's pores, heavily coat the surface with a mix of natural oil and paint thinner, then vigorously sand with 100-grit paper.

A watery mix of insoluble materials is a slurry. To Jim, that means an oil/varnish, such as Watco Danish Oil, mixed with sanding dust. "The Watco darkens the pores for contrast," he says. "I pour a liberal amount on the surface, then sand vigorously with 100-grit-the paper has to produce sanding dust."

With burlap, a towel, or an old washcloth, Jim packs the slurry into the wood. "I don't wipe off any excess slurry," he notes. "I just let it dry overnight. Then, I sand it again, adding more oil if needed. The new sanding dust blends with the original slurry and further fills the pores when I pack it in. This time, I wipe off the excess before letting the surface dry. After the second slurrying, all the grain should be filled."

The tinted oil in the slurry will have colored the entire wood surface. To color only the pores requires removing the dried surface oil with more sanding. "If you don't want to stain the wood," Jim advises, "simply use a clear or natural oil, such as linseed oil diluted about one-third with paint thinner. Then your slurry will take on the ambient color of the wood and tend to wash out the grain for an even look (see photo below). In either case, I let the surface dry for several days before final sanding and the application of a finish coat."

"Paste filler comes from the can the consistency of peanut butter. So thin it with paint thinner, benzine, or naphtha to a heavy cream," Jim instructs. "And because a paste filler is generally off-white in color, you'll have to add stain or a tint to it if you want to accent the grain (as shown below). Otherwise, the light-colored filler will obscure it. You can purchase colored fillers, but for better results, color your own."

Jim applies the paste filler to the surface with a small plastic spreader (as shown below), pushing the creamy material across the grain into the pores of the wood. After the filler has dried, Jim sands the entire surface with 120-grit to remove the filler from the non-porous areas. The remaining filler accentuates the grain. After cleaning the surface of dust with a tack cloth, he lays down a clear finish.

According to the finishing expert, a paste filler can provide some spectacular results. "You can stain the wood dark, seal it, then put on a lighter filler for contrast. Or vice versa. Myself, I like to make the wood jump out at you."