Cleaning old Guinness keg residue

[ytsejam]

I've got an old Guinness keg I've cut the top off of, and it has Guinness residue that looks kind of like rust.

Any suggestions for cleaning it?
The only thing I've tried is a long soak in hot water & oxiclean and that didn't help.
Would filling and boiling for a couple of hours do the trick?

 

[Dwain]

ytsejam,
check out this thread:
https://www.homebrewtalk.com/f11/keggle-cleaning-113060/
I finally got mine clean, but it was a BIOTCH. Luck - Dwain