when I did my stand I put three coats of killz inside and out. On the inside I used some latex exterior paint that was left over from the trim on the house. Paint seems to be holding up been has only been a year.
For the exterior, I took my wife to pick out the color. Talked to the guy at Benjamin Moore and asked him what the best paint would be. The last thing I wanted was the paint to peel off the stand the same way it did previously on the manufactured stand and the wall behind the tank. The guy blurted out I would definitely go with a Direct to Metal paint. So, we left home with some in the color my wife picked out.
Paint went on thin and was thinking no way is this going to cover. Think it was three or four coats later, which was no big deal because it dried fast, and coverage was good. I have been impressed so far with the paint, it cleans off easy, and protects the wood from soaking in any moisture. It's an acrylic paint so I worry more about chipping it off, then it peeling off.
Check it out, get a sample and try it on some scrap, then abuse it a little and see how it holds up. Paint is Benjamin Moore Super Spec D.T.M. (direct to metal) low lustre, think you can get it in flat and gloss as well