OEM, I believe the tanks were in the cab when it got the final paint ,so the exposed face and top were interior color - the undercoating job on yer old tank was the "tank renu" stuff they use when boiling out and relining a tank - if ya wanna paint the tank like original and it's galvanized, wash it down [outside only!] w/ 50/50 vinegar and water and the paint will stick - or just leave it galvanized finish, which looks good IMO
Bill
