You should always burn out a drum. Even if it's new and unlined. Fire is an amazing sterilizer. I'd take a sander to it and see what it does. If the sander doesn't appear to remove anything then you're probably ok with just a good hot burn. There may be a clear coat of some kind you're not seeing.
Pro Q Frontier, UDS (Little Red), "Turtle Drum" UDS, CG Gasser KCBS CBJ#61160