MDF, or medium density fiberboard, is composed of glue and sawdust. It is a stable, economical alternative to plywood that is making inroads in the cabinet industry. While it is partially made of wood fiber, it doesn't paint the same way. MDF does take paint well, but only if it is properly prepared. It doesn't deal well with water, so it is necessary to paint MDF kitchen cupboards to protect them.
Take the doors off the cabinets and remove the hinges and door pulls from the doors. Drawer fronts can be left on the drawers, just remove the drawers from the cabinets.
Use 220-grit sandpaper and lightly sand the surface of the MDF cupboards. Clean the sanding dust using denatured alcohol and a lint-free cloth.
Coat any exposed ends of the MDF with a sanding sealer. This will help limit the amount of paint absorbed by the raw ends of the MDF sheets and prevent them from swelling by absorbing too much paint.
Apply a coat of primer to the cupboards. Look for a primer designed for slick surfaces. This can be brushed with a paintbrush or rolled on with a 4- or 6-inch cigar roller. If possible, paint the tops and the backs of the cabinets also to help seal out airborne moisture.
Sand the primer once it dries, using 220-grit sandpaper. Remove the dust as before. Apply the 1st coat of paint just like you applied the primer. Use a latex semi-gloss enamel paint to stand up to the rigors that kitchen cupboards are exposed to. Sand the first coat with 400-grit sandpaper. Remove the dust as before.
Apply the final coat of paint. If desired, lightly sand with 600-grit sandpaper after the paint dries to produce a consistent finish on the paint surface. Install the door pulls and hinges and mount the doors on the cabinets and replace the drawers.