I just cleaned mine out today w/ Oxyclean - no residue after only an hour.
Here's my procedure:
- After racking, don't let it sit for an extended time, since you want to clean it while things are still moist.
- Swirl up trub with warm/hot water, dump (or harvest yeast if you want)
- Get out everything you can by swirling and rinsing a couple times
- Add one scoop or so of oxyclean
- Fill with hot tap water, at about 1/3 and 2/3 full, swirl it around a little
- Fill up to above the residue line, preferably close to the neck
- Let it sit at least an hour (residue should disappear)
- Empty / rinse / repeat rinse a couple times
- Turn upside down to drain and air dry
If you have really stubborn residue, some have said they add a small wash cloth or something similar and slosh it around with a gallon or so of hot water. The cloth will wash the gunk off. Just don't put too big of a cloth in or it will be difficult to get out. Do not use a brush or anything abrasive. (I have not needed to do this since Oxy works so well).