There are generally two ways to waterproof water-based paint. You can brush on a coat of simple clear waterproofing sealer, or you can stir in a paint additive before you begin the application process. If you choose to brush on a sealer, the process is as simple as dipping and brushing. However, if you want to go the other route, there are a few things you need to know before you get started.
Work in an environment above 10 degrees Celsius (50 degrees Fahrenheit).
Clean the surface you are working on thoroughly before applying the paint.
Pour the latex paint in a 22.5 litre (5 gallon) bucket.
Pour in the waterproofing paint additive in a 1-to-4 ratio. The additive should represent 20 per cent of the total paint volume, or 4.5 litres (1 gallon) of additive to 18 litres (4 gallons) of paint.
Stir the paint with a wooden stir stick for at least four full minutes. Apply the paint to your surface.
Waterproofing paint additives help to prevent mildew and mould by keeping the paint film dry, and most additives are compatible with all types of latex paint. These mixtures help to prevent damage caused by moisture, including damage due to rot, freezing and warping. They can also prevent colours from prematurely fading and help surfaces remain clean and free of dirt and grime. SEI Paint Proofer is a quality waterproofing paint additive which guarantees to provide 100 per cent water repellency. However, there are several different brands of additives available at your local paint store, including Rustoleum.
Never mix a water-based waterproofing paint additive with oil-based paint, as this will ruin the paint.