acid to remove rust in deep pits

What should I use for deep pits in the old roof. Someone said muriatic(sp?)acid? I have tried rustmort and have still had problems in the past. Any good suggestions?

 

3M Metal Prep. Mix 50/50 with water in a tin can. Use a 3M scuff pad and go to town.

Make sure you wear heavy rubber gloves and two pairs of latex gloves underneath those. It won't casue permanent damage (if used once in a while) to your skin, but it it up for a week or so.