Yes Adam, the trick is to make stitchwelds in a way that will not apply too much heat in one area because that will result in warping. Start welding in an area where the two pieces line up the best. While there is still heat in the area, the metal will be more maleable and thus you should be quick with the body hammer should the panels need some convincing. Then move to another area at least 3 inches away if possible and make your next spot weld. Continue in this fashion until your panels are stitched together every 3 inches or so. Now start over about a half inch beside the very first weld and continue tacking beside every successive weld so that heat buildup is kept to a minimum. Once this round of welds is complete, it might be a good idea to let the metal cool a bit. This is where you can go around with the body hammer to fit the panels where your next welds will go. The rest is pretty straightforward. Just keep going around making your welds until you have filled the spaces between them to about a half inch or so.
The next step is to grind down the welds but you must also be careful not to introduce too much heat into your work. For this I use a 4 1/2 inch angle grinder with a sanding disc of about 60-80 grit. Heat buildup is not as big a concern when grinding a part that has a bend in it like the door pillars or the lip below the rocker panel so don`t worry too much there but if we are dealing with a flat panel surface, grind the weld in the same manner as they were welded. Do a few in one area and move to another area away from the first, letting the metal cool before going back to a previously grinded area.
Idealy, you would have made the patch carefuly to minimise the amount of filler needed to smooth things afterwards but this is not always the case. If you have to build up an area by more than 1/8 inch, use short strand fiberglass and finish with body filler for a smooth surface. Any small pits that remain are easily repaired with finishing putty and sanded smooth prior to priming.
In the case of my 4Runner, I knew the paint was going to take a beating so I primed top to bottom front to rear with an epoxy primer/sealer. This type has better adhesive properties. (like epoxy paint for the concrete floor of your garage) It also seals better and has a tougher finish that is impervious to moisture unlike a scratch filling primer that can retain water and allow the paint to bubble after only a few years. This primer still needs to be sanded to achieve a super smooth surface but your topcoat will still stick to it very well even if it is not. This is handy for areas that are hard to sand or impossible to reach.
I hope this helps you out a little. I did work in a bodyshop one year to pay for university and there I learned quite a bit but I am far from an expert so if my techniques seem incorrect please feel free to tell me.
Toughguy
