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.

It’s a good idea to sand your porch before painting. This will remove loose or chipped paint and debris, and give you a cleaner surface so that your new paint goes on smoothly. But if the existing surface is in good condition, it’s not necessary to sand your porch before you paint.

In most cases, sanding an old deck before staining it is necessary to achieve a decent finish. If your old deck has a relatively new stain finish on it and you're only doing touch-ups, you can generally skip the sanding part. However, if your goal is to make the deck look and function better, sanding first is a must. 

Here's the deal with leaving used rags covered in oil-based stain lying around in a pile: Depending on the conditions, these rags can take just an hour or up to a few days to combust. The source of ignition is caused by trapped heat that can be compounded if the rags are near a heat source.