I would look at the available tech data. There may be a special aluminum primer. My guess is that it is required, topcoats don't like to stick to bare metal. Maybe someone with experience will chime in and help us avoid looking it up.
96 gary fisher 'utopia' : 99 Softride 'Norwester'(for sale), 1972 Raleigh Twenty. Surly 1x1 converted to 1x8, 96 Turner Burner
Anyway, I didn't clear coat it. But here is how it turned out. Usiing aluminum stock
and electric conduit. I put 5 coats of Duplicolor on it and let it dry for about 4
weeks. But I don't think the paint will stand up to normal wear and tear. Any hints on
how to give it a bombproof coat ?
Yes, primer is necessary. On aircraft the protective is stripped and the surface roughened up first, normally apply an epoxy based primer coat followed by 2 coats of enamel overcoat for all exposed surfaces. Works well for corrosion prevention and some impact protection. A good overview is http://exp-aircraft.com/library/alexande/painting.html
Please bear in mind that an assembly such as the rack would also have sealant between all mating parts to prevent fluid from becoming trapped in the joint.