rusty soft spot on tub side

I am in the process of redoing my guest bath, and just removed the shower doors and track. It all removed easily, but I've found a spot, about the size of a nickel, of rust. This is on the flat top portion, the area where you might sit to shave legs, or hang a bath mat over to dry. The spot was completely covered by the lower track before, and while scraping the caulk and adhesive away, I scraped away the caulk that had obviously been used to cover/fill this spot. It doesn't really feel soft, but is very uneven, clearly not just a stain. The tub is metal, maybe enamel over? which surprised and stumped my dad whom I'd asked for advice. Any ideas on what I can do to hide this ugly spot and prevent more damage. The tub also has a similar size ding in the bottom, and isn't worth refinishing, I'd replace it with a deeper tub before going to that expense. Just looking for a quick fix here. Thanks!