These penetrating liquids seal the wood so that it won't shrink, crack, or rot. But they contain little if any pigment that prevents surface graying. One type, which typically contains wax or silicone, makes water bead up on the surface; another allows water to spread out in sheets. "Beaders" degrade quickly, usually in a matter of months; you know it's time for a fresh coat when the beading stops. "Sheeters" can last a few years. Water dries faster on sheeters and doesn't leave behind spots like a beader does, but there's no tell-tale sign that it's time for a new coat.