Stains penetrate wood best when it's cool and dry, so you want to avoid moisture. If the wood is wet upon application, or if it rains before the stain dries, the water will soak into the wood and displace the staining solution. This leads to a flaky and blotchy look, which is the opposite of a sumptuous even coat. Once the stain has fully cured, after about 3 days, rain shouldn’t cause you any problems.

You can use latex or acrylic paint outdoors. Although acrylic paint tends to last longer outside, it also costs more, so if you’re painting the outside of your house, for instance, latex paint is often a go-to option. However, acrylic will be your best choice if you want to spend some extra money for added durability.

Applying too much sealer to a deck might seem like extra protection, but it can cause more harm than good. Wood is a porous material with a limited capacity to absorb liquids. When you go overboard with sealer, the excess can't soak in and instead forms a film on the surface. This leftover layer is prone to peeling, cracking, and trapping moisture, ultimately defeating the purpose of sealing in the first place. 

Wood stain does go bad, but it will last in the can for between three and six years, provided you use proper storage techniques. Storing wood stain in open containers or in extremely low or high temperatures will cause your wood stain to become unusable much more quickly. Water- and latex-based wood stains typically last for just two to four years, while oil-based wood stains can last for three to six years.