With any wood like pine or mdf, they will blotch. I would use a water based pre-conditioner, the one I use is General Finishes pre-conditioner. It is really time sensitive, so one has only about 10 minutes to apply the dye. You can also seal the mdf with a quick light spray of shellac from a can, and then apply your dye. I would use a bunch of test boards and find out what works for you.