Anything to fill rust spot in bathtub?

I've noticed a 3/8" rust spot in my old bathtub. Is there anything similar to the automobile rustkits that would allow me to fill in the spot with some sort of quick drying epoxy or caulk, smooth it over, with a clearcoat like paint?
I live in an old building and the bathroom sink had a larger rustspot. When I poked my pinky in it, it went right through and I don't think the sink is salvageable.
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I should add that it's a metal bathtub, or as described here:
http://www.diydoctor.org.uk/projects/bathrepairs.htm
it might be called a cast iron (enamel) bath", and the spot looks more like a chip right now than rust, but it's probabl slightly rusty. The site above mentions something called: "The Mr. Tubby DIY Resurfacing Kit".
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needquickbathroomrepair wrote in

there are small kits of epoxy for filling in chips,and kits to refinish the entire tub,much more expensive and much harder to do. Home Depot and Lowes have them.
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Jim Yanik
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Thanks for the info. I'm going to head to Home Depot and price it. If it's too much, I'll check the web first.
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