If you paint directly over a mildew stain, bleed-through will prove immanent. To permanently seal a mildew stain and prevent recurrence, you must prime the affected area. Unfortunately, ordinary primers won't permanently block a mildew stain. To effectively block a mildew stain, apply a special stain-blocking primer formulated to seal even the most stubborn stains. Kill the mildew before beginning the application process, or fresh stains are likely to present themselves at some point in the future.
Fill a plastic spray bottle with water. Add one teaspoon of bleach.
Spray the area affected with mildew with the solution. Let the solution sit for three minutes.
Wipe down the surface, using clean wet rags. Let the affected surface dry for two hours.
Apply stain-blocking shellac primer to the mildew stain, using a natural-bristled paintbrush. Let the area dry for two hours.
Clean the brush, using denatured alcohol.
Apply latex paint to the area, using a synthetic-bristled paintbrush. Let the finish dry for two hours.
Apply a second coat of shellac primer if the mildew bleeds through the first coat.
You must use diluted bleach to kill the mildew before beginning the application process, or you will likely see a recurrence. Ordinary latex and acrylic primers will not block a mildew stain. Mildew stains could be a sign of a water leak. Be sure there are no leaks before applying primer and paint, or a recurrence will prove likely.