Try mixing up a "hefty" solution of Iron Out in a large enough container. Set the tank in and weight it down. Let soak for a couple of hours and rinse it off real good. It does need to soak though; just wetting it won't do much. That does wonders on rusty surfaces, it may just work on this.
Good luck. The project looks like it's moving right along, very impressive.
Last edited by Snipe; 04-08-2013 at 08:23 AM.