I've used needle scalers in the past, work very good for knocking heavy rust off, never used one to remove undercoating; I have my doubts; I see the bottom side of the car being reshaped and/or ventilated.
Used the hell out of them in the Navy, the undercoat will definitely clog them if soft and you have to be careful on sheet metal. I use a heat gun on the undercoating. Think of a scaler like a air hammer with 20 chisels. They do work great on heavy ga metal.
