Set the desk onto a drop cloth in a well-ventilated area. Remove the drawers and any hardware from the desk.
Remove any flaking or peeling paint with a wire brush and gently sand the entire desk with 120-grit sandpaper. Wipe away the dust with a tack cloth.
Wipe down the desk with a rag soaked with mineral spirits to remove grease and residue. Allow the desk to dry completely.
Turn the desk upside down and spray a coat of primer on the underside of the desk using short strokes and holding the can 6 to 8 inches away from the desk. Let the underside coat dry and flip the desk over to prime the top of the desk.
Spray a coat of spray paint in the color of your choice to the underside of the desk, and then spray the top side when the underside is dry. Allow the paint to dry completely and spray a second coat of paint if the coverage is not ideal.
Replace hardware and insert drawers. Let the paint cure at least overnight in a dry area to ensure that the paint will dry to a hard finish.