Discovering ugly blotching after applying stain is one of the worst finishing problems a woodworker can encounter—especially because the unsightly patches of dark, deeply absorbed color seem to appear out of nowhere, without warning. This problem can be very difficult, if not impossible to correct. Treating the wood with strong bleach usually doesn’t work. Sanding until you remove all the unwanted dark patches is the only solution, and that can mean removing a lot of wood.
Woods like pine, fir, birch, maple, poplar, and cherry are among those known to be susceptible to blotching. The root of the problem is that these woods are unevenly porous, so they don’t soak up stain consistently across the surface of a board. This can be due to obvious changes in grain direction or early- and latewood differences in density, but there are also variations in absorbency that are hard to detect and predict. Wetting a board with paint thinner will sometimes reveal darker areas that are likely to blotch.