I have some rust there too...  Bastards!

Automotive touch-up paint will do.  Nail polish is laquer, and won't hold up.

I was thinking about getting one of those touch-up pens anyway...  I scratched my rear plastic when I was trying to take it off without taking off the rack...  (Yeah, I know I'm stupid... Thanks).

It looks like Color Rite are able to ship some touch up pens to me over here in AUS so I'll order up some primer, the light blue and some clear coat. That should (hopefully) get it back to a decent amount of paint on the affected spots, and won't look too bad, should also be useful to patch up some other scratches on the tank.