It is a shrinking disc, a flat piece of stainless that goes on a 9" grinder. I discovered it on YouTube and bought it here. The owner, Ray Schelin makes and markets these discs. I bought the disc and a backing plate that makes it more rigid. I did not bother with the DVD that goes with it. Ray was very quick and helpful answering my queries.
I hired a 9" grinder locally. I did try using my 5" grinder but it did not work at all well: the gyroscopic forces generated by the disc spinning at 10,000 rpm attached to a small grinder made it almost unmanageable. The bigger heavier 9" grinder spinning at 6,000 rpm was much easier to use.
Its purpose is to remove high spots by shrinking metal and I can say that is works very well. Using the disc as flat as possible you apply the disc until the high spot gets hot, then quench it with a wet rag. It is much easier to do this job with two people. The effect is amazing; the high spots just disappear before your eyes!
The results really doesn't photograph well but after about 20 minutes work I had achieved a result that I was happy with. It will need skimming with filler and I'm sure with more time and skill you could get it even better but I keep reminding myself that this is just an old truck and making it look like a classic car would be ridiculous.
