Maybe try a ranch supply store like tractor supply, also could look into industrial paints like are used on chair lifts. When I did installations for Lietner Poma, we used this heavy duty paint to touch up our tower terminals. Vary nasty stuff to apply, but holds up pretty well.
Also maybe whatever they use on off shore oil rigs, that has to be some good shit, dealing with salt water corrosion, might do well with the road salt.
" I wish I were a headlight, on a North bound train..."