Birth of a 1970 Dart Convertible

When butt splicing sheet metal there are a number of things that are important. As others have said make sure the metal is clean and freshly ground (even if it is new sheet metal it will have rust inhibitor or scale), and make sure the gap between the metal is even. I usually turn the heat up a bit on the welder to ensure good penetration during spot welding..if you can see the back side of the weld you should see the bead come through. I do a spot weld for about 1 second. Then I take an air hose and blow on the weld to cool it. Next I take a angled die grinder with a 60 grit mini disc and grind the spot weld flush. Be careful when grinding as you can make alot of heat and warp sheetmetal. Then I blow off the ground weld to cool it. Then I do another spot weld. The reason I grind in between spot welds is to ensure the next weld penetrates the sheet metal and the previous weld with no porosity or tiny holes. Tiny holes can cause paint bubbles in the future if moisture gets in from behind. When you are done the panel or area shine a flashlight behind the weld and look for porosity and repair as neccessary. Make sure you use some fibreglass bondo (water proof on backside and frontside of weld to try and seal it. just a thin coat but use the spreader to push it into the weld to fill and small holes you don't see. This process takes a while, but the results are good.

If you are trying to weld your frame connectors in, use the same spot weld process and aim the gun/wire more at the heavy gauge frame and as others said start the puddle on the frame connector and pull it over to the sheet metal and stop. I wouldn't grid these welds as they are on the heavier gauge metal. Also make sure you grind the frame connector to get the mill scale off.

Hopefully I helped you, good luck!