Aggressive boiler cleaning help.

[Nightforce]

I see lots of posts about cleaning runs of newly built stills but I'm looking for some help specifically with cleaning a keg boiler. I know that my still heads are clean, I've run them on a 1/4 keg boiler and they produce fairly clean water distillate; only a slight metallic taste to the water which is to be expected. My 1/2 keg boiler just won't seem to get clean. I've boiled a lye solution in it (1# in 15gallons water), a salt solution and multiple water cleaning solutions but there is still a slight presence of an off odor and flavor in both the water and alcohol runs. Does anybody have a good method to aggressivly clean a SS boiler. It was a really old keg that sat outside for many years before the person gave it to me. It seems fairly clean inside but I can't get rid of the the off odor and flavor that comes through.

Thanks.