GOOGLE painting cast iron claw foot tub
From the site:
Tub refinishing can cost an additional $500-$1000. Refinishing is a painting process that will repair defects in the porcelain. The interior of the tub is body-filled, sanded, acid-etched, primed, and painted. The exterior is sandblasted down to the original cast iron, and then primed and painted. The tub will look smooth and brand new again, but will not last nearly as long as a new cast iron clawfoot tub. In fact, a refinished clawfoot tub has to be treated much more gently than a new tub, so if you plan on using it on a regular basis, it would be much more economical to purchase a new clawfoot bathtub.
If I were going to paint the outside of the tub, I would consider epoxy paint. The material is a small cost. If the inside needs refinishing I would buy a tub that doesn't need refinishing.
If you can't sand blast to white metal it will soon show rust through the paint. You may be able to do a decent job if you use a primer that is designed to prevent the underlying rust from bleeding through.