Ultraviolet or (UV) rays destroy the lignin or "glue" that holds wood fibers together. If the wood is of poor quality, the sun can warp and crack it. Blockers are needed to protect against these rays. The more pigment a stain has, the more protection it will have against UV rays as well. The solid content offers the protection. There is some protection in clear stains having UV blockers, although naked to the eye, they are opaque to the rays.

There are two kinds of UV blockers available. One is a UV absorber, acting like an energy absorber. The absorber dissipates the energy before it can do any harm to the wood structure. Energy absorbers tend to break down quickly and offer little protection after that. The second one is transoxide UV blockers. These blockers contain metals that have been ground to microscopic size. When the UV ray hits the metal, it is reflected. A much more reliable protection for sure.