Usually when I finish a keg and open it up for cleaning, the sides of the keg are pretty clean and shiny and silvery and there is just a little puddle of whatever beer was in there before down at the bottom. Sometimes there is a little schmutz on the curved bottom. This has been pretty consisted regardless of the style or color of the beer.
I recently emptied a couple of mediocre kegs. On one I had been blaming on old hops, the other was a diacetyl bomb from being removed from the yeast too quickly. Both of these had a brown coating all over the inside of the kegs. The coating was a dark brown, not like beerstone. The kegs were both ambers - a Vienna Lager and an American Amber ale. The dark residue washed off with a sprayer, but not easily.
Anyone noticed a correlation between nasty looking kegs like this and...well, anything? Old malt, bad water, infections, anything?
