The smooth, level texture of the surface is an important feature of wood finishes. The normal tendency for a wood coating is to telegraph the texture of the wood through to the surface of the finish. This rough-textured surface of the finish causes light to reflect and refract in many directions, causing the finish to look harsh. A rough surface also obscures the vision of the wood below instead of enhancing it. The pores of open-grained woods such as oak, mahogany or walnut are among the most prominent features of wood that create texture. The pores absorb more finish than the surrounding wood, causing a depression in the surface of the finish above. In the worst-case scenario, these pores cause the finish to be rejected, creating holes in the finish.

Pores don’t always need to be filled to the point where the surface of the finish is completely level. It’s acceptable in most cases for the finish to have some indentations caused by telegraphing above the pores. The degree to which the surface is leveled is a matter of taste and affordability.  However, when the level and sheen of the finish contrasts strongly between the pores and the rest of the wood, the overall appearance suffers.

The best method of filling pores on open-grained woods depends on several factors:

• The size of the pores; this varies by species and cut (plain sawn, rift, quartered, etc.)
•  The type of finish used
•  The desired look of the finished product

Finish formulators strive to create finishes that will self-level as they dry. Ideally, a finish will “bridge” the pores and level itself over them to a certain extent. If a particular finish does this well and the pores are not too large, they can be filled by applying several coats of finish and sanding in between coats to help the leveling process.

This is not the case with most waterborne finishes, however. When applied by spraying, most waterborne finishes not only don’t bridge the pores, they actually form a ridge around the edge of each pore, similar to the edge a volcanic crater. This exaggerates the contrast between the pores and the surrounding wood.

Building up several coats and sanding in between coats actually makes the problem worse since each coat responds the same way and the chasms become deeper with each coat. The resulting finish resembles molded plastic made to look like wood. One way around this problem is to pad or brush the waterborne finish on, if that’s cost-effective. Grain fillers usually won’t resolve this issue.

In many cases, the pores are too large or the finish is too slow in filling the pores to use the process of repeated coating and sanding.  The best method in these cases is to use a grain filler. Two basic types are available: oil-based and water-based.  The oil-based products are best used on top of a sealer by applying them liberally to the surface. Rub off the excess first with a coarse cloth such as burlap across the grain and then with a finer textured cloth along the grain.

Waterborne grain fillers are applied heavy enough to fill the pores, left to dry, and sanded off. Some waterbornes can be modified to work in a manner similar to the oil-based products by adding retarders and lubricating agents. You have to work fast or they will dry and become virtually impossible to rub off with a dry cloth. A damp cloth will help but the process becomes more difficult as time passes and the filler will need to be sanded off if it gets ahead of you. The problem with sanding is that it’s difficult to remove the filler from corners and carvings and be careful not to sand through the sealer.

Grain fillers can be used to add interest to a finish by coloring them differently than the surrounding wood colors. Dark colors give an aged look, especially when some filler is left in the crevices. Light colors give a “pickled” look and some bizarre effects can be achieved by using colors not normally associated with wood.

Open pores in wood are not just a problem to be overcome; they add interest to the overall appearance of finished wood.  Even though they can detract from a finish if they appear as holes, they add interest to the color and even the texture of a finished piece when treated properly.

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