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.