Mild Rust Inhibitor?

I recently found a local place that has open top unlined drums, and I wanna build a uds. The person did say the drum was treated with a "mild rust inhibitor", and all I would need to do is clean it out with hot, soapy water. Does this sound correct to you guys? Burning it out is not an option because I live in a crowded residential area, but if need be, I can put a propane torch to it. I was gonna use the propane torch to burn off the outer paint actually. Any info you gurus could give me would be highly appreciated.

Thanks!