I'm a little late to comment on this thread, but I found a solution to this problem different than the above comments. My situation was a pink tub made from cast iron with minor rust stains. After researching several DIY refinishing kits, I decided to call a professional to refinish the tub in white. I shopped around for the best price, warranty and durable finish that I thought would do the job. Here is a link to their website
. They completely refinish the tub in place. It takes 1 1/2 days and it looks brand new. The process is not cheap, but it costs a lot less than replacing the tub and you can select over 20 colors (but white is the least cost color).
I don't work for this company. My business is rehab/flip houses. When business is slow, I also do small remodel/repair jobs. Since the pink tub house, I have used the re-finishing process on two other remodel projects where the owner had attempted to refinish the tub by himself, gave up and decided to remodel the whole bathroom. These guys are 3 for 3!