I don't own a spot weld bit. I use a die grinder with a 3" cut off wheel and grind the spot weld.
If your using a Blair or any other spot weld bit you usually use a center punch to get the spring point to stay center and not wander. The goal is to only drill close to through the first layer of metal leaving the second layer untouched. You then need to us a tool of some kind to detach the top piece of metal. That's the goal. After I had several of my guy's break the bits and toss them like they were free, and guy's drilling through both panels causing damage to the inner panel, I had them start using a die grinder.
The biggest issue is the drill through. Even if your not going all the way through and you score the inner panel thin you end up with a thin spot that will weaken the attachment point for your replacement piece.
I plan a how-to and will post it soon.
Here's the overview:
Tools I use: Die grinder with 3" cut off wheel. Screw driver with a reshaped end like a chisel. light to medium weight hammer.
First, never remove body panels on a collision damaged car until you have re-dimensioned the chassis. By cutting off a section of the car that is crushed you trap damage that would otherwise be repaired as you pull and straighten the damaged section. Unitized cars transfer damage well back into the car unlike the old frame cars.
Here is an example of what I mean. Page two shows damage repair. Notice I left the headlight buckets in place until I had the inner panels and outer fenders ball parked.
http://www.914world.com/bbs2/index.php?sho...04128&st=20Then when your satisfied you have things in reasonable shape cut away the damaged panel sections close to the spot weld flanges. This makes it easy to operate the grinder close to the flange. Carefully grind the spot weld marks to thin the metal. I only thin the metal. Then I carefully pry up the flange with the screw driver and start to split the flange off the inner panel.
Once you have competed this you will have spot weld bumps along the inner panel that you can now grind off. You end up with a completely undamaged inner panel ready to spot weld on your new piece.