Blotching is caused by stain penetrating deeper into some parts of the wood than in others. It’s common with woods that have uneven densities. Examples in soft wood are pine and fir. Tight-grain hardwoods such as cherry, birch, maple, poplar, aspen, and alder may also be prone to blotching. Aqua Coat Blotch Barrier prevents stain blotching by holding down grain raise and controlling penetration of the stain into the wood substrate.