I have many hours invested in using a shrinking disc. It does work to shrink the high spots but I have found it to be very slow going. You would use the disc if you did not want to use any body filler. I bought mine from one of the members on the site listed below. I don't remember his name for sure (John Kelly ?) but questions on this site will bring a response from him. He has a web site with some good pictures of his excellent metal work.

http://www.allshops.org/

Here he is www.ghiaspecialties.com