How to deal with Mold and Mildew Before You Paint

A common reason for re-painting is the desire for a fresh, clean surface. But simple repainting will not remedy the most common cause of paint discoloration: mildew. Find out how to remove mildew before you paint!

What is Mildew?

Mildew is the common name used to describe mold and its discoloration, which often appears as black, green, or brown fuzzy, powdery, or slimy patches. Since mold is a parasitic fungus its web-like body can eat through and into porous painted surfaces. Because mildew destroys the integrity of the paint film, moisture can enter the painted surface causing masonry to crumble and wood to rot.

Spores, the fruit or seeds of mature mold float easily and invisibly in the air seeking the perfect environment to root. Mold thrives in warm, moist, humid conditions with poor ventilation, particularly bathrooms, kitchens, and laundry rooms. Mildew can also be observed on exterior surfaces with condensation from poor air circulation and little direct sunlight, notably north-facing walls or walls shaded by trees or shrubbery.

The easiest way to tell if a surface is covered with dirt or mildew is to place a drop of common household bleach (5% sodium hypochlorite) on the soiled area. Without scrubbing, mildew will bleach out in about two minutes, dirt will not.

The Remedy

Once mildew is confirmed The Paint Quality Institute advises mixing a disinfecting solution of three quarts water to one-quart bleach. Because bleach loses effectiveness over time it is important to use fresh bleach in the solution.

Wear eye protection and rubber gloves when applying the solution with sponge or garden sprayer. Let stand a half-hour before rinsing completely with plain water.

To protect outside vegetation from the bleach solution rinse them with water before starting the application and cover them with clear plastic. To prevent heat damage, remove the plastic as soon as the application is complete and do a final water rinse of the landscape plants.
Because chlorine can damage some plastics, aluminum, or chipped enamel surfaces put it on a small test area first.

Finally, never mix liquid chlorine bleach with ammonia or products containing acids. Doing so will release toxic gases. Many common detergents or cleaners contain ammonia.

The Right Paint

Many exterior paints as well as bathroom and kitchen paints are formulated with mildewcides to make the paint mildew resistant. Mildew inhibitors such as the biocide barium metaborate prevent the growth of new mildew but they do not destroy the existing fungus. Paints with mildewcides should not be used on windowsills, doorframes, or any area where children could suck it or ingest it.

Paint with zinc oxide, which is a common paint pigment more resistant to mildew than paint containing titanium dioxide. Paints or primer containing linseed oil are highly susceptible to mildew as are lower grade paints commonly extended with thickeners, which can feed mildew.

Water-based paints, latex paints that have a porous finish such as a flat or eggshell sheen are highly susceptible to mildew. Glossier latex paints especially 100% acrylic semi-gloss and high-gloss latex paints, oil or latex enamel coatings are less porous so mold spores have fewer surfaces to grab on to.

Prevention

• Keep surfaces clean because dirt as well as greasy films from cooking oils and soap scum is food for mildew.
• Control moisture because mildew spores need water to germinate. Keep shrubs and trees away from exterior walls to promote air movement and drying. Consider using darker paint colors because they absorb more heat from the sun and dry faster after rain or dew.
• Install adequate insulation or heat rooms evenly to prevent condensation. Dampness will occur when warm air hits an inadequately insulated wall adjoining an unheated room.
• Use exhaust fans vented outside to remove the two-gallons of water a day that can accumulate in homes from cooking, laundering, and bathing.
• Paint on a calm day so those mildew spores do not blow into the new coating.