For you guys that have the "dreaded" red liner in your barrels. Burn about two barrels worth of oak pallets through your barrels and then paint or do whatever you are going to do to the outside. Then spray the inside down with Pam or some other spray oil and fire that baby up and start cooking. With the heat generated from 2 burns of oak pallets, there is no worry about any baddies left in the barrel that will leach out and affect any cook you might do in your UDS.
Think about it! When your up to cooking temp, the sides of the barrel on the outside is not hot enough to burn your hand if you accidentally touch it. It won't scorch the paint you painted the drum with, so it will not get hot enough to put out any bad fumes from any left over red liner. Besides, the pam will form a layer of soot and seasoning to the inside of the drum. My drum had the red liner and after two hot burns, I quit and did not sand, grind, or do anything else to the inside of my drum. I finished the outside and then started cooking on it. Two years later, I am still alive and well and my drum is like it was when I built it. (Of course I do have a second nose growing on my forehead, but I think it was caused by other outside forces.)