The long answer is: I used to paint on canvas, but the texture of the fabric started to bother me. I wanted the decision to be up to me, wether I needed a smooth or a textured surface. So I started painting on wood. I used to prepare my own (masonite) panels. Grounded them with Gesso (a Talens brand) and a brush. Watered the Gesso down in the first three, maybe four layers. Less water in each following layer and you avoid "gumming-up". I used very fine sandpaper to smoothen each layer, always with lots of water. To speed up the drying process I had a hair dryer on stand by. To tell you the truth: I hated it, it was an awful lot of work. The results were great though.
Now I don't ground my own panels anymore. I found a guy here in Holland who does a great job preparing MDF panels. Smooth surface like you've never seen. At first it was hard, painting on his panels. Very slippery, made me feel uneasy. But over time I found a technique based on layering that allowed me to do just the stuff I had in mind.
I did a 70 minutes video (Painting Clouds by Layering) that tells the whole story. Here's the trailer.