I have a chip in my tub - I believe cast iron - slightly more than 1/2in diameter. I had one repair man tell me to use a rust-inhibiting spray paint and then porc-a-fix and then someone else tell me I should only need proc-a-fix. I've attached a picture. The rest of the tub is in good shape and this is not near the drain, so I would like to repair it myslef. Any suggestions would be appreciated! Picture attached.