Even the toughest exterior paint or stain will probably fail before its time if it's brushed onto a dirty, poorly prepped surface or gets rained on before it's dry. Whether you hire a pro or are among the roughly 20 percent of homeowners who do it themselves, these tips from our experts will help ensure that the finished result looks better longer while safeguarding you from health hazards:

1. Inspect for lead

This is where to begin if your home is painted and was built before 1978, when lead paint was pulled off the market. The federal Environmental Protection Agency recommends having the surface checked for lead and hiring a pro to do the prep work. While children are especially vulnerable, anyone who comes in contact with lead is at risk.

2. Invest in proper prep work

Presuming there's no lead, make sure all loose or peeling paint is sanded or scraped off the surface. Thoroughly clean it with a scrub brush or power washer to remove dirt and chalking. Let it dry completely, which can take a week. Replace cracked, split, or rotted boards. If you're painting, be sure all bare wood is primed with the recommended primer for the paint being applied. Remove old, cracked caulking and fill cracks around windows, doors, and trim with acrylic caulk.

3. Get rid of the mildew

Left alone, mildew can quickly spread and turn fresh paint applied over it into a blotchy mess. In addition to cleaning the surface, be sure any visible mold is treated with one part bleach to three parts water. Treated areas should be rinsed after 20 minutes, washed with a detergent solution, and allowed to dry completely before painting.

4. Avoid color mismatches

You can get color differences from can to can based on the store and even who mixes the paint. Be sure to have all the paint mixed at the same time or buy it from the same batch codes. For the best uniformity, mix all the paint together in a large container, then return it to the original cans.

5. Check the forecast

Many paints applied in cold weather might not dry properly, resulting in poor adhesion. What's more, rain can wash paint right off the siding if it hasn't dried. Temperatures between 60 and 85 degrees with little or no wind are ideal. Also try to avoid painting in direct
sunlight, which may cause blistering. Be willing to postpone the job if necessary.

6. Choose the right brushes

Even with the right prep and paint, using the wrong brush can mar the final result. Synthetic or natural-bristle brushes are fine for oil-based (alkyd) paints. But stick with synthetic brushes, especially nylon, for latex paint because natural bristles are hollow and can clump as they absorb the water in latex. Synthetic bristles also tend to provide cleaner, more even brush strokes with latex. If you're doing the painting or staining, look for angled bristles for more precision around windows and doors. Be sure the brush fits snugly in your hand. And dampen bristles before use so they load the paint or stain more evenly.

7. Insist on a guarantee from the pros

In addition to the specifics of the paint or stain to be applied, any contract you're asked to sign should include the work being done and a clear finish date. Also be sure it includes the painter's promise to correct—at little or no cost—any chipping, peeling, blistering, or flaking that occurs within two years after the job is done. If the pro tells you the paint itself has a warranty, remind him that the paint warranty doesn't include labor, which costs far more.