Can Milk Paint be applied to raw MDF or masonite?

Yes.

MDF

1. Apply Kilz or Bin 123 Primer first. 
2. MDF is not as absorbent as natural wood, so wait 2 days in between coats and before applying top coat.
3. MDF also tends to cast a brown color. If you are using a white paint, apply a coat of SeaGull Grey first to counter the brown, and then apply a couple of coats of white over it, allowing all coats to dry 2 days.

Learn more about how to prep MDF before applying GF Milk Paint.

Masonite

Masonite is harder than MDF. It will also need to primed with a bonding primer like XIM. XIM is known for its high-quality primers that bond to "Tough-To-Paint" surfaces like porcelain, tile, glass, plastics, and more.