Basement walls aren't like the other walls in your house. They are part of the foundation, and because they lie below grade, they are subject to moisture produced by condensation and seepage. Failing to remember this when you paint them could result in peeling and bubbling shortly after the paint dries. You can't avoid a certain amount of condensation, and if the walls are seeping, you may need to improve drainage before you start painting.
Assess the condition of the walls. If there is a previous coat of paint, you'll need to remove it if you want to repaint with concrete paint, because concrete paint needs to adhere to a porous surface. If the paint is old, it may be lead-based. You can check by purchasing a lead-paint test kit online or at a home center. If the paint turns out to be lead-based, consider getting professional help in removing it, because lead-based paint is toxic. Look for areas of seepage and high moisture. You'll want to seal leaks by filling them with hydraulic cement, but excessive seepage is a sign of drainage problems that you need to address before painting.
Preparing the Walls
Scraping the old paint off the walls can be time-consuming, but it's a necessary evil -- do it with a paint scraper and wire brush. Once the walls are clean, look for efflorescence, the white salt deposits that accompany seepage. Clean them with vinegar or, if they are stubborn, with muriatic acid. Neutralize the walls after an acid treatment by washing with a solution of 1 cup baking soda per gallon of water. After you fill cracks by troweling in a mixture of hydraulic cement mix and water, you should give the walls a good cleaning with a strong detergent solution, such as a mixture of 1/2 cup trisodium phosphate per gallon of hot water.
Choosing Paint and Primer
If proper preparation is 90 percent of a top-notch paint job, priming is probably 9 percent. The primer not only seals the pores of the surface you're painting, it provides an adhesive base coat for the paint. The primer you use should contain a mildewcide to control fungus growth on the walls, and it should also be alkaline resistant, especially if you had to seal cracks with hydraulic cement. Choose a waterproof masonry paint for the topcoat. If your basement has poor ventilation, you'll want to stick with low-VOC water-based products, of which several are available.
Applying Paint and Primer
Because bare concrete soaks up primer quickly, it's best to apply it with a brush. You don't have to worry about streaks, but it's important not to leave voids, or areas without primer. Once the primer dries, the paint goes on much more smoothly, and you can speed things up by rolling it with a thick-nap roller, using a paint brush to fill in cracks and depressions and then back-rolling. Apply a second coat in the same way. Some efflorescence may leak through and become apparent after a few days, so save a small amount of paint so you can brush an extra coat on these areas.