you got to clean them really good with a solvent to get the oils and scaling off the steel, then use a good primer followed by a couple coats with a good enamel.  The more you put in it the longer they look nice.  Could just weld them up and spray them with a cheap enamel then plan to paint them every few years, I guess you either put the work in up front or later every few years sprayin them down.  Kind of like grain bins, do it right or do it often.