rust under bathtub "lip" seam. Caulk?

Whoops. Computer error.

Anyway, as I was saying...

My rather old metal bathtub was manufactured in two parts with the tub you sit in fitting into anotherpiece that has the exterior side panel. The area where the two parts meet is rusting. Water drips or condences on the top of the tub and runs down the exterior side about an inch and a half to the point where it curves inward and meets the exterior side piece. It is the latter piece that has the rust. At one time I caulked it, but I stopped because I thought it might actually encourage the rust, plus the rust discolored the caulk. But I guess I'll have to caulk it in hopes of keeping water from the area.

Should I sand the rust and get some kind of enamel paint like rustoleum and apply it with an artists brush, and then caulk? Any know how this should be handled? Hope I described it alright. With one piece fiberglass units being so common these days, most of you probably won't have a tub like I'm talking about.