I recently built an electric UDS, and I love it. But during the build I had the normal "HOW DO I GET THIS PHENOLIC LINER OUT?" issue. I had a media blaster, but it wasn't super effective. I don't live somewhere I could burn it out with pallets.
So I borrowed a propane weed burner from a friend. I hit that phenolic with as much heat as I could. It did not budge. The paint on the outside peeled off, the metal was glowing, but that dang phenolic just turned dark and stayed where it was. Some quick wire brushing showed it was still well adhered and this was largely ineffective.
So I figured I would use the weed burner to burn off all the labels on the drum and remove the original paint. I hit the outside with the flamethrower.
And lo and behold, the phenolic resin on the inside started peeling off. Viola! I spent the next half hour burning that barrel from the outside in, and when I was done, I literally just dumped the phenolic coating that had peeled off into the trash bin and admired my clean metal barrel. I think moisture or other volatiles between the layers must have vaporized and liberated the bond between the metal and plastic.
Your mileage may vary, but this was my experience. Propane weed burners are not expensive and have many other uses, like applying 3rd degree burns to the back of ones leg, lighting your fence on fire, and other general mayhem. Everyone should have one.