I am currently doing my front porch adding all kinds of recycled stuff outdoors. First MDF can not be used outdoors it will get ruined if it is exposed to water. Get a primer that can go over acrylic or enamel. I would take time out to research and think about the design you want. After priming I would use an exterior acrylic, they are water based, easy clean up. Sketch out what you want and take the time to paint it on the exsisting shutters. It all depends on the kind of shutters that you have. If you have the traditional ones you might want to just change them and build some more rustic looking ones. A couple of vertical pieces and two horizontal? How much work are you willing to do so they come out looking good? Good Luck!

Exterior grade plywood might work for what you want to do.

After painting the shutters, you could add a little more protection to the surface by coating with a spar (exterior) varnish ... comes in a shiny or satin finish.