I use a 9" SS shrink disc on a heavy 7-9" grinder, 6000 rpm to shrink and blend high spots on my panels. After a panel has been welded, it shrinks. You then stretch the weld to bring it back into line, but very often, sisnce it is easier to stretch than to shrink, things get over stretched. The shrink disc offers a nice slow controllable way to get the panel blended in nicely. Basically, this process is very simple. The disc spins across the panel, touching only on the highest spots, creating heat. Then you wipe or spray water with some soap mixed in to cool it. The water only serves the purpose of being able to get your hand on it to feel the high, because at a warm to touch temp, all the shrinking for the moment is done, untill you go across with the shrink disc again.
I try to give the panel a quick block sand to have my highs show up. This way I can see and feel them, and recognize where the panel needs work. Some will be small dents like I am going to work with here, others will be larger knots or general highs that need to be brought down.
Spray some water on the panel, the included soap will give some lubrication to help prevent galling, espically on aluminum panels. Then, take tour disc and keeping it flat on the panel, go across your high spot, slowly at first. If it is too high, some hammer and dolly work may be required. Going slow prevents a small sharp high from shrinking too fast in too small an area, creating a ripple around the high.
After a couple 3-4 passes, alternatly shrinking and cooling with soapy water (and a rag to help catch excess), the high starts coming down and blending in with the ajacent metal. I use a body file now rather than a sanding block to help me read the surface.
You can see that the high is blending in, and the spot that touches is getting larger.
Also, the more that the high is shrunk, the less effective the shrink disc will be. This is a good thing, because it helps prevent you from going too far.
Also, the body file can tell you alot. The file will want to cut the high, try to not let this happen because it will thin the panel too much if you arent careful. You can just feel that it wants to, and like the disc, after the high comes down, it wont want to cut anymore. The idea is to use it as a float file, just marking the metal in a cross hatch type of fasion. A good operated file wont lie to you.
We all will want to wipe our hand across the panel to feel it, and sometimes the simplest of things like surface texture will give us false information. This panel has some surface rust on it, and the area that I shrunk from being sanded, disced, and filed is smooth. Using a wet rag that you caught the excess with helps this, and it actually amplifies the feel of your hand. A handkerchief works best, nice and thin, This works as well.
The high is now blended away, ready for primer.
Keep in mind that you dont need to get the metal blue to shrink, and on many panels, if you do, it would be too fast in such a small area. I try to get it to just at or around the temp that the water steams off, that means I am going slow and careful.
Like any other process in metalshaping, the last 20 percent will take you the most time. Sometimes you dont need it "perfect" and thats ok, depending on your application.
This is juat a basic overview of shrinking highs on a nice low crown panel, probably the easiest to try. There will always be diffrent areas that get tricky, big overstretched areas, knowing what needs to be shrunk or stretched, reverse curves, what have you. Hopefully this helps people using their disc in the future.
Marty