I'm a good DIY'er with a good set of tools, but I have never worked with cast iron before. Since this is a small, localized problem it seems like a good place to start.
The drain in my seldom-used bathtub has developed slow drip at a rust spot in a 45-degree cast-iron elbow. The 2-inch pipe on either side of the elbow appears to be in good condition, although I can't be certain until I remove the elbow.
A close-up and a general view of the situation is at:
What's the best way to undertake this repair?