To find out how deeply these finishes would penetrate wood, we drilled 1/2-inch-deep by 1 1/4-inch-diameter flat-bottomed holes with a Forstner bit into red-cedar and lodgepole pine boards (we knew that penetration would be minimal in any tropical hardwood decking, so we didn’t test any ipe). Next, we poured a sample of each finish into the holes to a depth of 1/4 inch and let the samples stand for four weeks. Finally, we ripped the boards twice—once on each side of the hole—to give us a clear view of how far each finish penetrated into the surrounding wood.
In some cases, the finish was completely absorbed into the wood. In others, the finish dried to form a film. And in some cases, the stain didn’t penetrate at all; instead, a film formed over the top of the finish, leaving most of the liquid trapped beneath. We classified the samples according to what we observed, rating stain penetration as deep, mid, shallow, or film-forming.
In general, how deeply a finish penetrated the wood depended more on the species of wood than on whether the finish was oil- or water-based, or listed as a film-forming or penetrating stain. For example, in lodgepole pine, deep was 3/8 inch; in cedar, deep was more than 1 1/4 inches. Shallow in lodgepole pine was less than 1/8 inch; in cedar, it was 1/4 inch to 1/8 inch.