Old nasty keg!

I scored a keg today from a friend that I'll be able to use with my kegerator. Pretty sweet, right? Wrong. He had it outside for about a year is about 1/5 filled with beer that's just as old. I pulled out the dip tube and it was all crusty. Took are of that with an SOS pad. The inside of the keg is a different story. Since I can't really see inside, I filled it with soap and hot hot water and let it soak. After emptying it I filled it with hot hot water and Oxclean. Gonna let that set over night and see if it works. Any suggestion for cleaning the inside of a real nasty keg? Any chemical solutions I could use? I would just cut the top but it's one of those kegs with the rubber tops and bottoms...

If you are buddies with a local brewery, usually they have a system that will clean them. You drop off a nasty keg and buy a few pints in the tap room, next time you visit and buy a few pints you pick up your shiny clean keg.