Rust is cancer. The backside of rust is always bigger than the front. Cutting out and replacing is the best way to go for longevity and safety. You have to try and figure out HOW big the rust patches are by getting rid of the paint and finding out where the rust stops and good metal starts. After that cut past that to be on the safe side. If you have rust on your firewall you may have rust in other areas. Find the blockage stopping the water from escaping and check for rust? Rust is evil and is always proned to come back if it's not handled correctly.