Some people have peeled off the thermofoil and then used a shellac based primer on the MDF under it. If what's there is holding on for dear life, then you can always try scuff sanding the surface, using a high grip oil based primer, and then two coats of an acrylic enamel with high hide and adhesion properties like the new Behr Premium Plus paint.

I have w/ the method above. I used BM's STIX primer and BM's Cabinet Coat. (These used to be made by Inselx, but BM bought them.)

I'm also considering Carmel Colors. They claim their product adheres to absolutely everything. I keep reading great things about them.

I've been reading a blog by someone named Patty Henning, who has really put her heart and soul into instructions and how-tos. Here's the link to her shop. She says there's no tax if you're online, discounts on shipping, plus she gets credit for any sale. Her code is PattyH. She's out of Michigan, and being from MI myself, I felt supportive. :) Her store is Fabulous Finishes.

Disclaimer: I've not yet tried this method, nor have I any affiliation with this person. I have, however, read everything she's written and plan to purchase some of their toner for antiquing some furniture. I'm even considering painting my fridge.