Bleaching Mildew

"The wood siding and trim on our house was repainted just before we moved in two years ago -- but already the paint on the north side of the house is spotted and dirty. Is there a simple cure or do we need to repaint?"

Mildew, not dirt, is the probable culprit--and the cure may be as nearby as your laundry room, of all the unlikely places.

Mildew is caused by fungi that feed on nutrients in the paint film as well as any dirt on the surface. All painted surfaces contain plenty of nutrients for mildew to thrive, but only some surfaces have enough of the second ingredient--water--that mildew needs.

Indeed, moisture is the single most important factor in mildew's growth. Thus, mildew most often shows itself in places that stay damp--places such as window sills; the painted surfaces on or near gutters and downspouts; areas shaded by dense shrubbery or trees; and on the north side of buildings.

Mildew looks a lot like common dirt even you look at it up close. But there's a simple way to find out what you're dealing with: If you put a drop of household bleach on the surface, mildew will immediately turn white. Dirt, on the other hand, will still look like dirt.

How do you get rid of mildew? One time-tested remedy is to mix a quart of household bleach with a gallon of water and a cup of detergent. Make sure the detergent doesn't contain ammonia: Bleach and ammonia, when mixed, give off a poisonous gas.

The mildew should disappear quickly when you wash the surface with this solution using a medium soft brush, If you encounter any particularly stubborn spots, give them a second washing, this time doubling the amount of bleach in your mixture. Once the area is mildew-free, rinse it thoroughly with a stream of water from a garden hose. Then let it dry thoroughly.

What if your bleach test reveals that the gunk on your siding is dirt, not mildew? You should wash it the same way I've just described, except that you can cut down on or leave out the bleach.

What's next? If the mildew hasn't eaten too deeply into your paint, or if the paint was simply dirty, the cleaning job may allow you to postpone repainting.

If you decide to repaint, be sure to use coatings that are labeled as "mildew resistant." Assuming you've killed the old mildew underneath, these coatings will resist a new infestation much better than paint without similar protection.

Should you decide to wait another year or two before repainting, you'll have to clean the siding again--even if no mildew is present. Indeed, outside painted surfaces tend to attract a layer of dirt and grime from dust and pollutants in the air. While this grime (unlike mildew) won't destroy a painted surface, it will stop your new layers of paint from adhering properly--causing peeling.

Ken Holmes is an award-winning print and web journalist and editor, as well as a former contractor.