I just got a cool new tool based on some research in how to get rid of old rust and undercoating. It's called a scaler. It's used a lot in ships to get rid of scaly rust. It's just an air hammer with a bundle of needles - it's also called a needle scaler. It works really well. Pic 1 is the scaler Pic 2 is before Pic 3 is after about a minute.