not from a 'substance', simple result of tarping directly on the metal, condensation from our acidic atmosphere trapped and 'cooked' over time .... clean it well with a good brand 'wax and grease remover', sand it well, wax and grease remover again, prime and paint .... epoxy primer is best, but I'd get it all to bare metal for that

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics