more info please...what type/style of box? small medium, large? most of the time you remove rusted areas with sand paper then repaint with a paint designed for metals. smaller projects get spray painted, larger ones I brush on.
Medium size-Teflon coated base,having thick black stains,difficult to remove even with steel blades.
Spray down with WD40, wipe off. If rusty use WD40 scrub with steel wool.