Painting over waxed wood generally isn't recommended. Unlike paint or clear finish, wax -- even super-hard carnauba wax -- never completely cures. The solvents in oil paint will soften it, leaving a tacky finish, and wax prevents water-based varnishes from bonding to the wood surface, causing eventual peeling. Chalk paint is an exception; if the wax has hardened for a period of three weeks or more, you can apply it over wax.
Remove Soft Wax
Even if you're considering painting with chalk paint, you shouldn't do it over soft wax or furniture polish. To determine whether the wax is soft, try to scrape off a little with your fingernail -- if you're successful, the wax is too soft to paint and should be removed. Remove it with mineral spirits.
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Move the piece of furniture you're painting outdoors, if possible. If you can't do that, open the windows to provide ventilation. If you're sensitive to VOCs, put on a respirator.
Soak a rag with odorless -- or white -- mineral spirits and apply the solvent liberally to the painted surface.
Rub off the wax with a pad of 0000 steel wool. Keep the pad moist by dipping it in a bowl of mineral spirits, and change it for a new one when it gets encrusted with wax.
Moisten another rag with mineral spirits and rub down the surface after you've removed most of the wax with the steel wool. Check the rag for discoloration and rub again if you see any. Continue until the rag remains clean when you wipe.
Wipe the surface with a dry rag. It's now ready to paint.
Use this procedure to remove hard wax if you're planning on painting with latex or oil paint.
Painting Over Hard Wax
Although it's safe to paint over hardened wax with some brands of chalk paint, it's important to check the recommendations on the container for confirmation. Do not use the product if the manufacturer advises against it. If the wax is hard enough to paint over, etch and scuff it before painting.
Mix a solution consisting of 1/2 cup of trisodium phosphate per gallon of water. This creates a strong cleaning solution that will remove all the dirt and dull the wax finish to ensure better adhesion.
Wash the surface with the TSP solution, using a sponge, and rinse it with clear water. Let it dry.
Lightly scuff the wax, using 150-grit sandpaper. This is additional insurance against peeling and separation.
Apply the paint with a paintbrush, using long parallel strokes and stroking toward the wet edge of the paint. In most cases, you shouldn't need more than a single coat, but if you do, let the first coat dry for the recommended time first.