alright another question, when i fit the panels together, they are flush, but through the welding process the seam dips inward creating a valley between the two panels, anyone have a video for a little hammer dolly work to get that smoothed back out?
Some hints to start with..
Welding sheet metal with a Mig involves what I refer to as "dot" welding. One zap at a time.
Before you even get to welding, this starts with panel fitment, you want gaps as absolutely tight as you can possibly manage. Every time you weld a mig dot, it is going to shrink in a circumferential fashion, pulling from all sides. If you leave a gap, there is a better chance than not that the panel is going to shrink together more as the gaps pull together. On low crown panels such as quarters, door skins, hoods, etc.., this means that as the shrink brings the gap together, it is removing some of the crown that supports that panel. Planishing out welds is enough work as it is without adding having to compensate for panel movement in the mix. This is a sure fire recipe for a nice big oil can in the middle of the panel. So, tight gaps will eliminate some of this work. The panel will still shrink at the weld, but it will be nothing like you'll see with panel movement.
Welder set up... You should see a full penetration weld after each dot, meaning a slight bit on the back side as well, no panel joint visible. Use practice pieces to get your welder set up dialed in, but insure you don't lay it on a steel welding table as that will become a heat sink. I like to duplicate conditions that will be present on the vehicle, so a practice piece should be in mid-air with no backing, just like most panels on the car. So I normally tack practice panels together with a couple tacks and then span it across the jaws of an open vise. This should give you as close as possible what you'll experience when you do start in on the good stuff on the car. Now undoubtedly you will have a good chance that you'll blow some holes. If you have the heat set high enough for full penetration and are blowing holes, then add just a bit more wire feed speed. If there isn't enough filler going in for the heat applied, it will burn away the parent metal. So try more feed speed before jumping on the "too much heat". Once you get a happy medium, if you see your welds are too large, both front and back, try to limit weld sizing with shorter zap times.
Once your panel fitment is perfect, and you're ready to weld in the panel, you need to start your tack at one end, skip a few inches, and tack again, skip same distance, and continue around the panel, aligning the surface of the two panels as you go. Many people will tell you to skip around to keep heat buildup down, and I have been one of those. But to demonstrate why this process needs to be clarified, if you were welding a lower door patch (for instance) and tacked either end, you have a much better chance of panel misalignment than if you move progressively across the panel, aligning the panel surfaces as you go.. Panel misalignment, especially from tacking one end and then the other, will result in dips, buckles, etc. that will not come out. So the more we can do for better panel fitment and alignment will reduce the need for excessive filler usage. Continue with the same sequence until the full length of the seam has been "tacked" in place. Then go back and planish each weld dot one at a time, in the same sequence as they were put in. Then use a 3" cutoff wheel to grind down the welds to just above panel surface. This gets them out of the way for planishing the next sets of weld dots. After completing the grinding, weld dots in between the first sets, repeat the planishing and grinding phase. When the weld dots are spaced at about 2" apart or less, revert from welding between the previous to overlapping the weld dots somewhere between 1/3 to 1/2. The overlapping will help to prevent missed spots and help to eliminate pin holes. Weld one cycle, planish, grind, repeat. On a weld seam I like to save the sanding flush for the end. If you sand flush after each individual dot, you will have a better chance of inadvertent sanding of the parent metal to the sides of the weld, resulting in thinning the panel.
Now, having said all that, any time your panel is pulling into a valley, it is from the crown of the panel shrinking as the weld and HAZ cools. Think of the panels profile as an arc, and when the weld and HAZ shrink the length of the arc, it starts to become a straight line, hence the weld pulling the adjacent panel into a valley. This is why it is important to planish each weld dot after you weld it, while isolated from others. Each shrinks circumferentially, pulling from all directions. By planishing each weld dot one at a time, you are more effectively stretching the shrink back out, to help negate the valley effect you are seeing.. Doing them one at a time gives you plenty of room to work and insures the planishing stretches back out that one weld dot. Then grinding it just above flush gets that last one out of the way so planishing the next one that overlaps it will be stretched in the same "isolated" fashion. I don't have any videos (yet) on the planishing of the dots, but here is one on grinding down the welds. Where this is done on a plug weld, the grinding down to just above panel surface is identical, the sanding afterward on a weld seam I usually wait until the end, as described above..
