Wash away mildew stains before painting your surfaces.
Mildew tends to form on surfaces that remain damp and/or receive very little or no sunlight, for example, bathroom walls, ceilings, and windows. If the mildew is left on the surface and simply painted over, it will come through your new paint and ruin your paint job.
To remove mildew, wash the surface with a solution of 3 parts water to 1 part bleach. Use a scrub brush on larger surfaces or a toothbrush on smaller ones, such as the grout between ceramic tiles.
Let the water/bleach solution sit for 10 minutes and rinse with clean water. Make sure to wear protective eyewear and gloves.