I've been soaking my fermenters clean with 4TBSP of PBW in it,filled to near the top with water. Soak it for maybe 5 days & all the gunk disolves &/or falls loose to the bottom.
Now here's where it gets contradictory. I got an e-mail awhile back from beersmith about cleaning brew stuff. They say not to soak any fermenters in PBW or oxyclean for long periods,as it soaks into the plastic. I call BS on that one. Or maybe it's outdated thinking?!
I let my FV's sit for about 5 days,brush inside very lightly with the bottling brush to be sure everything's loose & falling to the bottom. It's sitting on the fermenter stand all this time. So I place my homer cheapo bucket on the floor below the spigot to drain the cleaned fermenter. Dump the old PBW/silty stuff down the head. I thoroughly rinse the fermenter to remove any cleaner left inside the FV. Then rinse that out with starsan & dry before storage. I clean & sanitize the spigots & mounting hole on brewday,& sanitize the whole thing right before filling. I've never had any cleaner residue problems with this method.