I've been having trouble with patch panels flattening out/shrinking when I grind them. I wrote Pete at South West Rod and Custom to ask if he had any advice for me. He said the heat from the grinder will shrink the metal just like welding will. He suggested I use a 3" cutoff wheel to grind rather than my 7" grinder. Also he suggested that I concentrate on grinding ONLY the weld.
I made sure to only use the edge of the grinding wheel rather than using the face. I also ran my hand over the work area every minute or so to make sure nothing was getting hot. I made a big effort to keep the wheel moving over the length of the welds rather than concentrating in a small area. I literally took one swipe on one line of welds, moved to the other side of the panel, took a swipe, moved to another area, etc. It made for about an hour and 10 minutes of grinding, but the panel looked perfect when I was done. The alternative is spending an hour beating the panel back into shape. Case in point; I'll be spending a few hours replacing a patch on the passenger side that I ruined by grinding too fast.
Thanks to all who thought nice things about my compressor... I don't want to speak too fast, but I think she's gonna make it. I did some reading on the interweb and found that it was just a loose belt. I tightened it up and changed the oil... She's running better than ever now!