Why Paint Peels
Paint shouldn't chip. It should bind to the surface below for decades with proper surface preparation. Without proper surface preparation, however, paint failure -- chipping from knocks or humidity -- is almost inevitable. In the case of humidity, excessive moisture in the air penetrates the paint and lifts it from the surface, such as the ceiling above a shower or the walls of a humid basement.
Homes built before 1978 are often finished with lead-based paint, which is hazardous if it's disturbed by scraping or sanding, and also when it deteriorates naturally. If you suspect the paint contains lead, or you're unsure, hire a certified risk assessor or building inspector to test it, advises the United States Environmental Protection Agency.
Dealing With High Humidity
Deal with excessive moisture, bringing the humidity level down to about 50 percent, so newly painted areas won't be likely to chip or lift again later. Ways to do this include running exhaust fans when you're showering or cooking, or installing a dehumidifier or proper venting in the basement.
Prep, Prime and Paint
It isn't necessary to remove all the paint. As long as you get the compromised areas smooth, and the low spots or holes level, you can achieve good repainting results.
Scrape loose, chipped paint from the surface, using a putty knife or wire brush. Wipe away remaining dust and bits with a damp rag.
Fill deep areas -- caused by removing several layers of old paint, for instance -- with joint compound, using the putty knife's blade for small repairs, or a drywall-mudding trowel for large ones. Allow the compound to dry for the time suggested on the label, which may be several hours or even a day or two.
Sand the compound -- or low, scraped areas -- smooth, using 120- or 150-grit drywall sand paper. Wipe off the dust with a barely damp rag. Run your hand over the sanded surface to check for flatness.
Apply a few thin or sparse coats of primer to the sanded areas with a paint roller -- as with paint, a few thin coats of primer adhere better than one or two thick coats, decreasing the chance of eventual chipping.
Allow sufficient drying time between applications; refer to the can's label, which may suggest 10 or 20 minutes between coats. For a relentlessly humid home, allow at least twice as much drying time for better paint staying power.
Apply a few coats of paint to the primed area with a lightly loaded roller, using a paint color that matches the surrounding hue. Again, allow sufficient time between applications.
You can apply oil-based paint over water-based paint, but not vice versa -- this would cause paint to lift, unless you start with a clean surface and an oil-based primer.