At least a couple good hot burns, soapy power wash an fer good measures a run with a wire wheel then another good soapy wash should do the trick.
Many a pig cooker been made from fuel oil barrels. Just be real carefull with the first fire, although diesel ain't near as explosive as gas er some other things, ya just don't wan't a surprise. Eyebrows er hard ta grow back without alotta explanations! Ya could actually power wash it first then start yer burns.