Cleaning Mold on Exterior Surfaces Before Repainting
When preparing a wall or painted trim for repainting, producing a stable and clean surface is important. Since you will be repainting, removing mold with a heavy-duty cleaner, such as trisodium phosphate and bleach is appropriate.
To mix, add 1/4 cup of TSP and 1/4 cup bleach to 1 gallon of very warm water. Apply with a garden type sprayer, regular spray bottle or sponge. Scrub the infested area and allow the TSP to sit on the surface for a few minutes. Rinse with warm water.
TSP and bleach are non-reactive. This means you can add up to 1 cup of bleach per gallon of TSP solution, for moderate infestations. Be aware that this will only remove mold and stains on the surface. Heavy deposits or growths will require an additional step.
Recommend wearing protective eye wear and clothing. Strong TSP solutions can burn the skin.
You are dealing with living microscopic organisms. You need to use of a strong bleach and water solution for serious mold issues.
Mix the chlorine bleach to water at 50% strength (that a 1 to 1 ratio). Add this solution to a garden sprayer, spray bottle or cleaning bucket. Purchase a new hand sprayer to prevent cross contamination with different chemicals that reduce the effectiveness of the bleach. For example, using an old window cleaner bottle causes some of the chlorine to form chlorine gas. Instead of killing the mold, the chlorine burns your eyes and damages your lungs.
If you do decide to use an existing sprayer, such as a window washer spray bottle, thoroughly rinse the bottle and pump with water to remove any residual chemicals.
Spray the mold contaminated area with bleach and water. Keep the area wet for several minutes, this will allow the bleach to soak into the pores and kill the mold. You can even allow the area to air dry, but be sure to follow up with another spray of bleach and water, and rinse the area down thoroughly to remove all mold and dirt.
Allow the area to fully dry. Then prime with a oil-based stain blocking primer. Now you can paint.
Some exterior house cleaning solutions have mold killing chemicals. These are a good alternative to the TSP/Bleach mix. Many of these cleaners can be injected through a water pressure washer for easier cleaning.
Choosing Exterior Paint for Mold and Mildew Prone Areas
Most exterior paints already have anti-mildew formulations. Special mildewcide additives are also available. These will reinforce the anti-mildew properties of the paint. These mildewcides will not affect the color or sheen and these additives are designed to inhibit the growth of fungus reducing the need to be constantly cleaning up mold on your house.