FYI, here are the four stages that I used to paint my tubes:
Stage one: Prime it. (actually, skipped this step and Deunan reminded me that I should have done that first. Ooops.)
Stage two: Mix these acrylics together: black, a dab of white (so it's not so jet black) and thistle green, to give it a little bit of a gunmetal color.
Stage three: I mixed burnt umber with burgundy so that it was a little more red than brown. I took my brush and wet it and brushed the entire tube, kind of like a watercolor, very light. I emphasized the rust a little more in a few areas.
Stage four: Lightly sprayed a satin finish clear coat on top. I tried gloss and it was way too glossy. Flat was way too flat. Satin seemed to have the right finish.
So far it doesn't show any signs of cracking but I'm sure I'll have to touch up some scratches here and there.