I have cleaned my ufm strapping it to a cement mixer, filled with diesel an a handful of nuts and bolts. It's been cleaned out 4 times and rumbled on for the best part of 8 hours in total changing the tilt many times.
I believe I rinsed it at least 5 times but never achieved a perfectly clean state as I could still scrape some crud of the metal with a long screwdriver.
Fitted a manhole cover, filled it with wheel cleaner stuff at a local car wash to the brim. Let it work for 15 minutes and then in with the hot jet washer for a total of maybe 3 minutes.
You could now eat out of it.
Cheers Bernd