Whether you are painting an exterior surface or an interior surface, you must first remove and treat mould and mildew. The process is virtually identical for either mould or mildew. There are a few basic considerations to adequately remove and treat the mould and mildew.
Identifying Mold and Mildew
Before you paint over a surface, identify whether you are painting over mould, mildew, or both. According to Nwrennovation.com, mildew and mould are the most common cause of paint discolouration. Mildew is black, green or brown and has a furry appearance. Mold tends to look more like dark spores. You can tell if you have mould or mildew if you apply a 5 per cent bleach solution to the discolouration. If it bleaches, it is mould and mildew because dirt does not bleach. If you notice either mildew or mould on the surface of a wall in or around your home, do not simply paint over it. Remove the mould and mildew first because painting over it will only cause the mould and mildew to eat away at the new paint.
Eliminating the Problem
After testing for mould and mildew, remove them. According to Nwrennovation.com, mix a disinfecting solution to remove the mould and mildew. An ideal solution is about a quart of bleach with three quarts of water. Gently apply the bleach to the affected parts of the wall or surface you intend to paint. Always wear rubber gloves and goggles when handling bleach. It is a severe eye and skin irritant. If you are working outside, protect vegetation with a plastic cover draped over them. Remove the cover shortly after cleaning and killing the mildew and mould. Keeping plastic over plants for an extended period will damage them.
Proper Paint Practices
If you are painting an exterior surface, consider using paint with a "mildewcide." Mildewcides are made with biocide barium metaborate and prevent the growth of mildew and mould. They do not remove mildew and mould, though, so do not rely on this type of paint to eliminate an existing problem. Avoid using mildewcide paint inside or near doors and windowsills because it can harm children who may inhale the vapours or eat the paint chips.
Other paints contain zinc oxide, a white pigment effective at resisting the growth of new mould and mildew. Avoid using paint with linseed oil because it is particularly ineffective at resisting new mould and mildew growth. The same is true of water-based paints.