The trick is to trim your patch panel as big as you want it. Then you fit it, overlaying the surrounding metal, then trim the surrounding metal to fit the patch.
Look at this way, picture putting a patch where there is nothing wrong with your original panel. Picture laying this patch over the bottom of a fender for instance, just laying it over this perfectly good fender. Clamp this patch up tight where it belongs over the bottom of the fender, do you have a picture of this in your head? The patch is covering the bottom of the fender and is held in place by clamps. You now mark the fender right at the top of the patch all the way across. One way is with a very sharp awl, that is what I like to use. So you have this patch clamped to the fender in perfect position and you mark right along the edge of the patch on the fender with your awl, right up against the edge. Remove the patch and cut the fender off BELOW that mark a little bit, say about an eighth inch or less. You then can trim that edge to perfection with a grinder with a fine disc like a 50 grit. You grind that down to perfection until the patch will fit up against the fender as it did before and you have a perfect fit just as you did before, now you weld it.
Another method is to do the same, clamp it in then cut thru both the patch and the fender with a 1/32" cut off disc. This will give you almost as perfect of cut as the previous example. I don't use that method unless I am putting a backing. If you are running a backing behind the seam you are welding you want about a 1/16" to 1/8" gap between the panels so you are welding ON the backing with both panels being welded to it at the edge of the seam.
Ok, so now transfer these same methods to a fender with a rusted hole at the bottom, works exactly the same way. The only difference being you may need to cut the rusted area away before you fit the patch because it is in the way of the patch being clamped in tight. So, this is how I do that, I set the patch into place and mark about where the patch comes on the fender, then cut the fender off below that a good inch or so. This allows me to get all that rusted metal and junk out of the pinch welds and so forth before I do the fitting of the patch. The fitting of the patch goes over that inch or so I left and then after clamping it in there where it belongs, after all the massaging to the patch , the fitting of the fender on the car so I know it's right, after all that I trim the fender to match the patch.
Notice the most important part in these two examples, that the panel is clamped in PROPERLY where it is going to be. Not sort of there, not "good enough until later" not anything but PERFECT as you will be leaving it. With it clamped in there perfect is the only way to mark and make the cut. Anything less than clamped in there perfect is going to cause you trouble. If this patch needs some massaging to make fit better as every repro part needs, you do that BEFORE you weld it in. You do that BEFORE you cut the final cut. You clamp it in after you KNOW you are ready for that final cut.
Last edited by MARTINSR; 04-30-2013 at 07:47 AM.