That liner is hard to remove. I made 6 UDS smokers this year. The first three had the liner, the last three I was careful to find barrels without liners.
Regarding the liner, I burned, wirewheeled and sanded. But still in the areas where the walls of the barrels met the bottom, I could not remove the liner to my satisfaction. I ultimately had them sandblasted for $25 a piece. Good luck with the liner.