Like most things, painting prep is important. With metal you can opt for solvent cleaning, manual cleaning (like wire brushes, wire wheel, sanding etc.) or you can go with several different grades of sandblasting (brush blast to white metal blast). The prep helps anchor the paint, and the cleaner you get the metal you reduce the chances of a failure (like rust through, lift-off, flaking etc.)
Temperature also makes a difference. Warming the metal drives off moisture reducing rust, and the paint dries faster and more even. You can warm up your drum with your weed burner from the inside.. You want a soaking heat, but only warm to the touch from the outside.