Basic toothpaste is also awesome for getting rid of rust on those weights in sea ponies (like it works for tail rust on normal ponies), but I'm glad your pony wasn't all rusty - most are pretty bad!!
I would definitely recommend oxyclean/Vanish oxyaction or similar to make sure there are no nasties growing in there as well. The nail polish is new to me, but sounds like a good idea
if the weight is really bad you can try and sand off the rust after cleaning, or if it's beyond salvaging then using a marble like it suggested on the restoration site is also a good idea.
Ooh the sea pony shown on that site with the bad rust, the nasty weight inside and then the after pic - those pictures are mine, and I'm credited on there for them
heehee I forgot about that!! she was a good example of how a reeeally bad condition sea pony can still clean up nice
(edited for typos!)