Figure out if your wood is treated or raw. Raw wood doesn't need anything but primer and paint. However, treated wood requires more steps. If the wood is already painted or has a glossy finish it's treated.
Pour the tri-sodium phosphate onto the cloth and wipe it over the wood surface to remove the finish on treated wood.
Sand the wood with a grit between 150 and 200. Sand the entire surface of the wood to remove any remaining finish and leave the wood raw.
Apply a paintable water-repellent preservative over the wood to prevent it from absorbing moisture. Allow the wood to dry for two days.
Paint over the repellent with an oil-based primer in a shade similar to that of your paint. Paint over the primer with one to two coats of acrylic latex paint.