Medium density fiberboard (MDF) is a composite material made by pressing together wood waste. This wood is often painted and sealed in such a way that any repainting efforts become exceedingly difficult. If you come across MDF that needs painting, like cabinets, bookshelves or tables, you can get the job done easily as long as you keep a simple routine in mind. This preparatory method provides a very deep and thorough finish on your painted MDF.
Scuff the entire surface of the MDF with a palm sander and 120-grit sandpaper. Don't use excessive force, you just need to create texture on the surface to grab the paint.
Wipe the MDF down with a tack cloth to remove all of the sawdust and prime the entire surface with non-latex primer. Leave the MDF until it is completely dry.
Sand the MDF again and wipe off the sawdust. Prime the sanded MDF once more and leave it to dry.
Sand the twice primed MDF, tack the dust off and then apply the first coat of paint in an even layer.
Leave the first coat of paint to dry, then sand it lightly. Once you clean off the sawdust, you should apply the second coat of the paint colour.
You can use the sanding between layers method of painting for any surface to create a better bond for each coat.
Don't attempt to sand the primer or paint if it is at all wet or it will gum-up the sander and mess up the wood.