Back in the 70's I worked at a Dodge Dealer that was across from a Steel Mill.
The fall out played havick with paint. Red ash embedded in the paint.
Scrub with Bon ami cleanser then wash with powdered tide and kerosene/diesel fuel.
Back before aluminum horse trailers the manu.of steel trailers recommended washing them with a solution of water/kerosene. This is back in the 60's,70's. They were like everything back then, painted with single state enamel.