Javeryh:
For MDF painting, I highly recommend you:
- sand the MDF while wearing a protective mask
- apply a coat of primer (Killz or other are fine). Use oil- base for oil-based paint, water-based for latex paints. Apply via roller or brush, not via spray can.
- sand, with mask, using 120+ grit sanding material
- apply second coat of primer
- sand, with maks, using 220+ grit sanding material.
This will get the sides very smooth and protect them against water and moisture.
For paint, use quality paint. Go to a real paint store, not Home Depot or Lowe's, and tell them what you are trying to accomplish. A good paint store should be happy to help and sell you a high-quality paint that will self-level and perform well. You may pay a bit more, but it will be worth it. They should be able to tell which application method is best matches your desired results with their paint.
Modern latex paints are fine as long as they are high quality. They are much easier to work with than oil-based paints.