Before rotten wood can be painted, it must be treated with a wood hardener. The hardener absorbs into the rotted wood to provide a firm base for the paint. If rotten wood is painted without this preparation, the wood will continue to rot and the paint will peel off. Painting over rotten wood should only be done if the rot is minimal. Large areas of rotted wood should be replaced entirely before painting begins. Exterior trim work and wood siding are likely places where rotten wood should be repaired and painted.
Remove all crumbling and rotten wood until you reach a reasonably solid wood. Dig the rotted wood out with a screwdriver. Don't be afraid to make the hole bigger. Vacuum out all loose debris.
Shake the can of wood hardener and then open the lid. Dip the disposable soft-bristle brush into the hardener and apply it to the rotten wood. Apply several coats of hardener, layer upon layer, until a shiny surface is achieved. Wait four hours for the hardener to set up.
Open the can of wood filler and scoop out a golf ball-size amount with a putty knife. Squeeze a 3/4-inch bead of wood filler hardener over the wood filler and then mix the two together until blended. Use the putty knife to mix.
Fill the hole with the wood filler until the hole is flush with the surround surface of the wood. Squish the putty into the hole by placing it on the broadside of the putty knife and then squishing it into the hole. It's OK if a little bit of filler overlaps the rim of the hole -- in fact, it's better if this happens. Allow the wood filler to dry for 24 hours.
Sand the repaired area with a 100-grit sanding block and then a 120-grit block until the surface is as smooth as desired. The area should blend in with the surrounding surface so that it won't be noticeable when painted.
Open a can of stain-blocking primer paint and dip a paintbrush into the paint. Paint the repaired area and then roll over it with a mini roller to smooth out the brush marks. Wait 24 hours and apply a second coat of primer. Apply two coats of wood finish paint after the primer has dried. Apply these two coats 24 hours apart using the paintbrush and mini roller.
It usually takes more coats of primer and paint to get a good finish on rotten wood. A large roller can be used for large areas.