Are the manual impact drivers which you whack with a hammer any use at all with anything but screws and small sockets? For example I need to shift a moderately rusty 15mm bolt which won't budge, on solid, flat bodywork - would one have any chance of helping? Some have a 1/2" drive so it sounds like maybe they can be reasonably beefy. It doesn't need to be fast and I'm aware it's not going to beat a powered driver.
