[mr48chev]

It's removing the Tar like substance (old undercoating) that runs up the cost when you have someone else do it. It has to be scraped as you can't normally blast it off. I've spent a number of hours with an industrial strength putty knife attacking that stuff. You have to be real careful if you use heat on it as it usually burns real well and catches fire easily as it is often primarily tar on the older cars and trucks.
The material used may reflect in the cost too plus the place I am planning on taking my cab to only lets one guy do the sheet metal body parts. The flunkies sandblast the heavy duty stuff with multiple coats of paint and what not but only one experienced guy does the sheet metal parts there.