needle scaler

I presume you're talking about a conventional needle gun. If so, I don't see this working well for you. They will pound the heck out of auto sheet metal. They are primarily used for chipping slag off of heavy steel components that have been joined by stick welding. Also, heavy rust that forms in layers on thick steel. Undercoating is relatively soft and doesn't 'chip' off. You'd be better off suiting up with a painter's mask, full face shield, leather gloves and jacket. Hit it with a 4.5" grinder with a heavy duty twisted steel brush (check RPM rating). Use cone and regular shaped brushes as necessary. It's messy but works rather quickly.