The rust can be removed, by several methods. The question is, what is under the rust. If the cooking surface is heavily pitted, I wouldn't waste my time.
There is the manual method where a wire brush on a drill motor is used. Labor intensive.
The vinegar method, where a concoction of distilled vinegar and water are used to break down the rust. If the piece is left in the vat too long, you will damage your piece. The vinegar will erode the cast iron.
The absolute best method is electrolysis.
The processes can be found on line, at IDOS, Camp-cook, and the WAGS site.
I think I would use the vinegar method. I have also read where alfalpha is used in the process as well.