welding outer sill

Hi there, could I ask how a new outer sill should be welded onto the car. Is the new panel lap welded and does the new panel go under or over the existing rear quarter. Thanks

The new sill should be joggled so the sill sits underneath the existing 1/4.
Once fitted fit a few self tapping screws along the join before welding as you won't be able to clamp the two pieces together.

Hi Yomp, thank you for your great answer. I am going to use mig brazing to limit distortion. Would I be correct in using tack/spot welds spaced out across the length of the seam and then slowly join them up by working back across?

I would drill some 6mm holes in the original part of the 1/4 in between the self tapping screws - say every 50mm but not through the new sill so you can see the new sill through the holes once its fitted and then you can whats called "plug weld" the 2 seams together through the 6mm hole. This will help when you weld the edge and get to the point where the self tappers need to come out and their holes welded to.
I would start one from the middle then one each end and then slowly weld in the middle of the welds alternating from side to side. You must let the panel fully cool before welding the next spot so it does take time. If you have a compressor with an air gun you can quicken the cooling process by blasting compressed air onto the weld but you must let it harden first or you just create a plasma cutter or sometimes a damp rag
The key is to keep the heat out!!
Another important - more important part in fact is to get everything fitting to the best of your ability before going anywhere near it with the heat. The tighter the fit, the less gaps there are to have to try and fill with weld which is when you get lots of heat into it.

The easy bit is the long flat edge. Its the bits that go around the door shut and arch which take the time and you won't be able to joggle those so easily and getting the line right on the door shut but I guess its how far up the quarter you need to go. Just because you have a big new full sill it doesn't mean you have use it all! I try to keep as much of the original car as possible - within reason.

 Its a bit like paint work - the more preparation you do before hand the better the result.