Remove all the desk's drawers, and then remove and set aside the drawer pulls and any other hardware. If the old white paint is in good condition, the desk will require no more prepping than a light sanding with 220-grit sandpaper to help the new paint stick. If the desk’s paint finish is chipped or flaking, however, use a heavier 100-grit sandpaper to smooth the rough edges. Carefully vacuum or wipe off all traces of dust and grit with a clean, damp cloth. Although preparing a desk for repainting is tedious work, taking your time with this step will ensure that your paint goes on smoothly and remains resistant to chipping once dry.
Use a paintbrush to apply one coat of primer. You may prefer to purchase a spray-on primer. Since your goal is to change the desk from white to brown, look for a primer in a darker color such as gray, rather than white. Even after priming, the desk will still look very light prior to painting. The Frugal Girl website recommends applying several very thin coats of primer rather than one heavier coat. Follow the product’s directions regarding the amount of drying time needed before painting.
Use a clean, dry paintbrush to apply a light coat of mahogany brown paint. Allow it to dry for at least two hours or as directed on the paint can. Apply a second light coat, again allowing it to dry. Since you are changing the desk’s color from white to dark brown, chances are that it will require a third and possibly even a fourth coat.
Once your newly repainted piece is completely dry, you might wish to apply a coat or two of clear polyurethane to the desk's top. Polyurethane helps protect and preserve the finish and is particularly useful for desks in children’s rooms or other places where furniture receives heavy use. Replace the desk’s drawer pulls. You might wish to purchase new hardware to give your freshly painted desk an entirely new look and style.