It’s not all that common, but sometimes you come across furniture, cabinets or woodwork that has a wax finish, and nothing else. There may be cases where you want to coat over with a more water- and scratch-resistant finish. How do you do this without having to go to the trouble of stripping the surface?
The first step is to remove most of the wax. Do this by washing with a mineral-spirits, naphtha or acetone solvent. Wash, don’t just wipe. That is, soak a cloth or paper towel with the solvent and wet the wood well. Then dry it quickly with a clean cloth or paper towel. Do this several times, until there is little or no wax residue coming off on the cloth or paper towel.
There are some situations on old furniture where the wax is resistant to the solvent. For these, scrub with #000 or #0000 steel wool along with the solvent. That is, wet the surface and the steel wool with the solvent and scrub. Then quickly wipe dry.
Once you have most of the wax removed, let the surface dry, then apply a coat of dewaxed shellac. The commonly available brand is SealCoat from Zinsser. You can also dissolve your own shellac from dewaxed shellac flakes.
Brush or spray on the shellac and let it dry for several hours, or until you can sand lightly with fine sandpaper and raise some dust. Then you can apply any finish on top.
The shellac is effective at blocking what little wax remains so it doesn’t interfere with the drying or bonding of the finish you’re using. Be careful not to sand through this shellac coat.