Cleaning the paint off your barrels so they don't offgas into the house and make you sick is an important part of building a Rocket Mass Heater. We've done it with a sand blaster (long and messy). We've paid a barrel refurbisher to do it (very nice job but expensive at up to $40 per barrel). We've thrown them in a bonfire (inefficient and dirty but fun, just don't breathe and don't roast marshmallows). We've used an extra lid for the drum we are trying to burn off to make a more efficient "pocket rocket" heater. That works but it doesn't burn all of the paint off evenly. Unless you wrap the barrel in newspaper soaked in clay slip the paint just goes directly into the air, and the basic pocketrocket it doesn't help getting the paint off a drum lid (we use them in some of our designs).
Enter the "overage barrel". These are special 60 and 80 gallon barrels one of my suppliers sometimes gets that are used to contain a leaking 50 gallon barrel without opening it to pour its contents into another barrel. So take one of these big barrels and make a pocketrocket out of it. Put three bricks at the bottom to hold your 50 gallon drum. If you want to burn off a lid you can put the lid on top of the bricks, then add three more bricks, then put in your barrel. Size the feed tube length so it sits a couple inches from the bottom of the inside barrel. Add fuel and go. We are finding that the big pocketrocket approach burns the paint off much faster, with less fuel because the outside barrel holds the heat in. The paint fumes also appear to burn up in the hotter environment.