Although water resistant drywall -- commonly referred to as greenboard or blueboard because of its color -- is available at most home improvement stores, you may find yourself needing to waterproof your current drywall. Most paint manufacturers make oil- and latex-based primers and paints designed to be waterproof and even mildew-resistant. These materials can be used to make your drywalling projects in basements, bathrooms and other potentially humid rooms waterproof so you don't have to spend extra money on pretreated drywall.
Clean the drywall with a shop vac or microfiber tack cloth. You just need to get rid of any excess dust that might interfere with paint adhesion, so there is no need for water and detergents.
Allow the drywall to dry. Some paints work just as well on dry or wet surfaces, so check the label to see if you actually need to wait for the wall to dry.
Pour the oil- or latex-based waterproofing primer into a paint tray. Roll the primer onto the drywall in a thin, even coat. Paint the primer onto small areas or in corners with a paint brush. Allow the primer to dry completely.
Pour the waterproofing paint into a clean paint tray. Roll the paint onto the drywall and use a paint brush for small, difficult-to-reach areas. Allow the paint to dry. If desired paint on additional coats until the desired effect is reached.