I have a similar barrel with the red liner that was used to ship grape juice. I did a burn for about 4 hours and even warped the bottom and the red paint did not come off. I have tried to use an angle grinder with a steel brush to grind it off and have decided that I now have a garbage barrel instead of a BBQ.
Someone gaive me a barrel used to ship Cetane, a diesel additive that has the capability of explosion if it gets too hot! I filled it with water and liquid detergent and cut off the top with a saws all. It has no liner but when it burned the Cetane lit up and got so hot the paint on the outside was even on flames. It got real scary for a minute with so much flame. Cetane smells a lot like rubber cement. We had to hose it down to control the burn but it burned up anything flammable and now l have a clean barrel. Luckily the other barrel's lid will fit perfectly so I believe I have all the parts needed to finish my UDS.
I think you are in for a long day if you want to continue with removing the red liner. I don't know exactly what is in the red paint but it holds very well and it is used with a food products so one has to wonder if it is safe or not at 300 degrees. On the other hand, I wouldn't recommend using a barrel with Cetane in it either. That stuff is bad but sure burned out the barrel nicely.