My first pit is a 120 gal compressor tank, old and rusted on the inside. After making the door and FB I burned a lot of wood in it to burn off the oil, rust and anything else in/out of it. Then took pressure washer (after it cooled) to it and cleaned it well. Burned more wood to dry it. Next day I got bacon bits and pieces (cheap stuff), the tank was hot from the sun, I smeared the fat all around the insides as much as I could, smeared the racks well. Then left the pieces to "cook" with wood for a bit to practice the heat settings.
Looked great, Started to actually cook ribs and let the insides built up of smoke. No issues.
Rust is iron oxide, same stuff they use in iron pills.
Good luck.