Many masonry paints can be purchased outright with waterproof capabilities. If you have already purchased paint and want to add waterproof properties to it, there are commercial products available for you to do so. These additives can provide 100 per cent water repellency, are compatible with latex paints, prevent mould and mildew and can even improve the fade-resistant properties in your paint. Proper surface preparation is an important part of the effectiveness of water-repellent paint on masonry walls.
Remove the old paint, if any, before attempting to waterproof. Waterproof additives may not be effective when painted over other paints or sealers, and warranties may be voided.
Examine the surface of your masonry for signs of efflorescence---a white crystal-like deposit often found on newer masonry. These are a form of salt deposits and should be completely removed before applying waterproof paint.
Clean surfaces with warm water and soap, scrubbing with a hard-bristled scrub brush to remove any build-up or efflorescence. Rinse well with clean water.
Check the masonry for any holes that need to be patched. Use a quick-setting hydraulic cement filler to patch holes and smooth evenly with surrounding surfaces. Allow to dry completely before painting.
In a separate container, stir in one part waterproof paint additive such as Paint Proofer to four parts latex paint, mixing thoroughly. Give the paint an occasional stir during the time you will be painting to keep it properly mixed during application.
Paint directly onto masonry surfaces with a good quality nylon brush, working into the pores of the masonry with the brush. Apply at a rate of 75 to 100 feet per gallon. Let dry for 4 hours before applying a second coat.
The second coat may be applied by a roller or spray gun for quicker application. Always paint when the wall and air temperatures are above 10 degrees Celsius and preferably during a dry spell.