I wish I could help... but I have never seen one with paint on the inside. The last one I did had some kind of coating as it was a food grade drum but it burned off albiet with some pretty big fires.
I would try burning again with wood. Get it really cranked up so flames are coming out of the top of the drum. If that doesn't get it then I would just look for a different drum.
Best of luck...
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Smokin Gators BBQ Team
Medium Amazin' Blue Spicewine - It's a Swamp Thing
Large BGE - Adopted
Medium BGE - Adopted
Weber 22 inch w/mods
Weber Genesis Silver C Gasser
Constant wood lurker