Should oil, spirit or water based stains be used on Medium-density fibreboard (MDF)?
I really like using a sealing primer like Bin's  Bulls Eye. this is a pigmated shellac based primer that can be sanded smooth as glass. It cures very hard, blocks any stains well and is an excellent base for topcoats. For a top coat, you can use a couple of coats of good grade latex gloss or semi gloss (personal preference).  If you want a really hard protective finish, let the latex paint cure for a couple of days and overcoat it with several coats of WATER  based urethane.