Hold on, "hammering metal like wood is a bad idea" is way too broad. Sometimes you need that aggressive approach. Trying to gently tap a bent steel bracket back into shape gets you nowhere. You need a heavy mallet and real force, @ivan827. It's about using the right tool for the job, not just a different touch. A body shop isn't a jewelry bench.
That's a rough way to learn the difference between wood and metal. Grandpa's tricks don't always translate to the machine shop. Feel for the guy, though.