I've never cleaned the inside walls of my UDS. It has a thick layer of built up gunk on it, top to bottom. Could be my imagination but I think all that gunk helps insulate the drum and maintain steadier temps. My UDS seems to perform better, the dirtier it gets. :)
....UDS....Mini-WSM....Midi-WSM....Mod-ECB....22.5-OTG....34"wood fired pizza oven....