Sharkhide ---- but only AFTER it's polished to where you want it... or put it on when the piece (radiator) is brand new! I use it on the rig - and many big rig guys with tons of polished aluminum use it to keep the stuff bright and shinning. It's expensive - but a little goes a long way.
It will NOT polish the pieces - and will dull them just a bit - so if you want perfect super polished show quality --- then don't use this - or at the very least TEST IT on something before you apply. It's a PROTECTANT not a polish.
http://www.sharkhide.com/