Painted wood surfaces, such as siding, decks or patio furniture, need a barrier from moisture. The optimal method is by adding a layer of moisture-resistant primer to the wood before applying a finishing layer of paint. The experts at Home Improvement Super Pages state that "the problem is that wood is a notoriously thirsty substance, and it is easy for water to get absorbed into the grain itself." For wood that's already painted, adding a layer of wood sealant is the easiest method for waterproofing. Larger projects can take a couple of days to complete, but the added protection is necessary to combat moisture damage.
Mix 60 ml (1/4) cup of liquid detergent with 4.5 L (1 gallon) of water in a bucket. Scrub the painted wood surface with a cleaning brush dipped in the soapy water.
Rinse with clear water using a garden hose. Allow the surface to dry for two to three hours.
Apply an even layer of silicone-based or polyurethane wood sealant using a paint sprayer. These products are available at all home improvement centres. Give the sealant product 24 hours to completely dry.
For complete moisture protection, it's best to strip the paint, and add a primer coat. Clear coat sealants won't last as long without a layer of primer coat beneath.
Wear safety glasses when using chemicals, such as sealants. Work in a well-ventilated area to minimise the fumes.