Mildew is a problem that's more prevalent in some areas than others. But, since mildew feeds on water and dirt... it can grow and spread anywhere these conditions exist. Here's how to test to see if a stain is actually mildew. Take some bleach and wipe it on the stain. If it comes off, it's mildew. Bleach is the only thing that will kill mildew. So, as you can guess, the answer to this problem is to remove all traces of mildew by washing with a solution of one part bleach mixed with 4 parts warm water. After washing with the bleach solution, remember to rinse thoroughly. Then you're ready to re-paint the surface. This should control the problem, however, despite best efforts, in some areas mildew cannot be totally prevented. Paints are not mildew proof... they are mildew resistant.