I posted this in a thread but thought I'd add it here. No I dont sell these but Ive been using one for years. Its a magnet with a spring and guage that lets you guage how thick the filler is in a car. When you get used to using it you can tel if theres multiple layers of paint from panel to panel.
I think its a must have for anyone buying an older car, and the the price is right.
http://www.carestoration.com/spotrot.htm