Mount the drum in your steel stand using something other than the bolts you intend to use so that you don't paint into the threads. You're mounting it so that you can get access to all sides of it. Put washers on both sides of both the frame and the drum so as to less damage anything when rotating the drum.
Use wood filler or putty to fill in the divits made by staples in the MDF, the joints between MDF sheets, and small spaces between the MDF skin and the rings. Fill up those bits with putty, let it dry, and sand it smooth. Just a single sanding should be fine; you'll cover it with a very thick paint later.