Burn it out, or sand blast it out and you will be good to go as long as the inside is down to bare metal. The local powder coat shop here charges me $25 to sand blast barrels inside and out. Another $50 to powder coat the exterior with high temp powder coating.
I wash mine, burn it out, then run a twisted cup brush on a grinder all around the inside until shiny. Then a final wash and dry out.
I i had somone near to sandblast for $25 I'd probably go with that.