Q. When I am installing a patch panel, how do I grind it and have it look like it was never even touched? What if there is no room to get behind it and use a hammer and dolly?
Via the Internet
A. One of the hallmarks of high-level metalwork is having welded joints that are finished so perfectly that they are undetectable. Often, the greatest difficulty is working out the distortion that welding can cause, and the flatter and thinner the metal, the more likely it is to warp. With good access to the backside, a skilled metalworker can work out distortion, but your control is much more limited if there is no access to the backside.
Your question, as I understand it, is more about how to smooth the weld and make it invisible, rather than straighten the distortion that the welding may have caused. There certainly are situations where the distortion from welding will be minimal. Welding a small patch into a highly curved portion of a 16-gauge steel firewall (like a Model A) is one good example of this.
The heat from the weld is what causes distortion in the metal, and unless you're careful when grinding down a weld, that can cause distortion, too. Having said that, thousands of skilled metalworkers around the world successfully sand welded joints and make the welds invisible. How is this done?
First, the bulk of the weld is knocked down with an aggressive abrasive. Some people use a 3/32-inch cutoff wheel in a die grinder to shave the top off the weld. These discs are quite rigid and can be used with precise control. I prefer using a sharp, 80-grit cloth-backed sanding disc with a hard backing plate, which could be 7-, 5-, or 3-inch, depending on the situation. Note that I said SHARP. I can't over-stress how important the sharpness of the disc is. Dull sandpaper causes a lot more heat, and you lose control over exactly where you are removing metal. When working on delicate material, such as 20-gauge steel, you have to be extremely careful not to thin the metal next to the weld, since it's only 0.035-inch thick to begin with. As you are removing the top of the weld bead, watch the area carefully, and stop immediately if you see any color change in the metal. Steel starts to turn a light "straw" color around 400 degrees, and that's hot enough to cause distortion. If you see blue color, you've most likely caused some noticeable distortion already.
Once the majority of the weld is shaved down flush, or nearly flush, with the metal next to it, you're "over the hump." Start using finer and finer abrasive until you get the finish you want. I often use a 5-inch dual-action sander for this, but I lock it to become a disc sander, with no orbiting. This allows me to use the convenient sticky-back paper discs, which are easy to change. I usually start with an 80-grit paper disc on the DA (which is effectively finer than the cloth-backed disc used initially) and then switch to 120-grit, and that gives me a very smooth finish. It's important to change the direction of sanding with each change of grit so you are cutting across the older sanding scratches at 90 degrees. Once the sanding scratches from the coarser abrasive disappear completely, you're done with that grit. Switching the DA sander back to orbital mode will help you blend the newly sanded section in with the rest of the panel, making it pretty much invisible. You can use even finer abrasives if you want more of a mirror finish, but for anything that will be painted, I find 120-grit leaves a gorgeous, satin-like finish on the metal.
I have a YouTube video that shows this process pretty well. Go to YouTube, enter "Ron Covell" in the search, and look for the video titled "Slicing and Dicing."