i use spray polly. mdf seems to suck up primer like a sponge. and with a good urethane primer it can get expensive. coating it with fiberglass resin works excellent too just a little harder to sand smooth. you can also mix some bondo in with some fiberglass resin, mix it up to a good sprayable viscosity, add mek hardener and spray it out of a paint gun. it will lay flat so less sanding, and easier to sand than just straight resin, kind of a poor mans spay polly