Does the air valve screw out? It may not be sealed well. If it is a pipe fitting, then taking it out and then putting on some pipe dope and (maybe) teflon tape may seal it. Could it be leaking from the threads that hold the valve in place?
It could also be that there is a gasket/washer/nut holding the valve in place. If so, can you tighten the nut?
Was there any rust under the paint? You might have some pin-holes.
I'm not a pro, just trying to help with some ideas. Maybe someone else can help. Although, answering these questions may make it easier to find the real solution.