Hydrochloric acid is the go. Works a treat and doesn't damage the tank. Just use pool grade acid and leave it sit for 10 or 15 minutes.

I dunno how the radiator joint cleaned mine, but they did a pretty good job, theres only a few small spots that are still brownish.

i cleaned mine with 'spray and wipe' ( you can buy it from bunnings $8 for one litre of concentrate) i just filled my bottle up with water added 2 cap fulls of S&W, shook it up a bit and left it for a day or so, then shook the crap out of it for a couple of minuits i had to do a few times but came out like new... just check out my gallery pics u'll find it in there (Black ED XR not the EL Wagon)... good luck with it..

Oh S&W is also really good for cleaning seats and interiour trims and stuff, well worth the money.. IMO..