For Mig welding, I use a systematic process in welding patch panels that actually promotes some cooling. Let me use some pictorials to explain a bit better.
Let's say we are installing a lower door skin to repair some rust. The repair panel has been trimmed for zero gap, and the panel is tacked on, starting at one end, and working progressively toward the other. Don't skip around from one end to the other as you have a greater chance of misalignment in that you may have more metal on one side than the other, which will result in a buckle from the excess metal on the one side. Start at one end, tack, move a couple inches, align the panels together, make another tack, repeat. This insures the panels are correctly aligned as you work progressively from one end to the other:
Once done with the last tack, go back to the beginning and use a hammer and dolly to planish each weld dot, working in the same start to finish pattern. Now go back to the beginning and use the 3" cutoff wheel to grind the weld dots down to just above flush, both FRONT AND BACK, as this gets the weld prouds out of the way for planishing the next successive sets of weld dots, and also removes all the excess weld that is going to act as a heat sink and affect your weld penetration. I grind to just above panel height for a weld seam like this, and final cleanup with a 3" roloc sander will be used at the end to dress the seam to the parent metal. Here's a video that shows the grinding process, but as this is a plug weld it is dressed immediately following.
Now that those welds are down and out of the way, let's add the next set. Here I'll overlap the last set of welds by about 1/3 to 1/2. Whatever the distance of your overlap, keep it consistent throughout.
This method helps to eliminate any missed areas like what may occur if you kept skipping around. Again, start from the same spot you did before, overlap the first weld dot done, go the next, overlap, repeat, until you get to the end. Then go back and repeat the planish from start to finish, then grind weld dots from start to finish, both FRONT AND BACK, and then repeat the overlap process again. Keep repeating the process until the weld seam has been finished, then use a roloc sander to dress the little bit of remaining weld to flush with parent metal, both front and back side.
If this sounds like a slow, monotonous process, that is the intent. This promotes consistency in the welding process, from fit-up through to final welds dressed. At no point is it necessary to cool the welds because they are too hot, at no time do we need to heat the welds up to planish.
Planishing per Ron Covell is to flatten or straighten out a bumpy panel (if I remember that correctly). As it applies to metal shaping, one would use a blocking hammer and a shot bag to add shape to a panel, but once complete it would appear as if it had walnuts all across the surface of the metal. A planishing operation would smooth these out to a more consistent "flat" finish (where flat means the panel has some crown to it, but no more lumps). As it applies to welds, planishing is the use of a hammer and dolly (or other appropriate devices as the job dictates) to add some stretch back into the weld and HAZ for the purpose of negating any of the shrink that has occurred. I would add that seldom will you get it perfect the first time as you go through the weld process as described above. You will most always have a bit of planishing to complete after the welds have been dressed out, or even some shrinking if you got too carried away in your planishing efforts.
