Not to pick on your method, but this is another good discussion point....
Spotting sheet metal closes the gap quite a bit where I need to keep opening up the space with an .035 wheel to keep it from warping.
All steel will shrink after the weld cools. From 19 gauge to 1" thick steel plate, IT ALL SHRINKS. The downside to gaps is it allows the panels to pull together, in effect you are losing some of your crown and creating a low. Continually cutting a gap with the .035 wheel shows it continues to pull the panel with each subsequent weld, creating more of a low as you progress, especially if you continue to open up a gap.
Like on that picture above, you can see the metal already with no gap whatsoever, so when that weld is finished, its probably gonna have to be hammered flat again. To me, thats the bigger deal, once it gets tacked and the two sides meet, continuing to weld is going to start warping a panel..
The No-gap was intentional, especially with a roof panel, we don't want lows pulled downward that will be visible at eye level. Having the tight joint keeps the crown that you have in the panel, and keeps it consistent. All welds shrink so we should be planishing regardless.. I find it easier to planish the MIG welds while they are all by themselves, then grind to just above flush. This also serves to keep the panel as close to original thickness so your welder setting remains consistent for the same thickness metal. MIG dots left intact with all that extra mass (up to about 5X the panel thickness depending on technique) would be more of a heat sink and cause that cold weld that stands up taller.
More heat, slightly more wire feed to prevent blowout, and much less time on the trigger pull should get you flatter weld dots, for easier planishing and much less grinding. Practice with some scraps the same thickness as the panel you're working on to get the machine dialed into the sweet spot for flatter weld dots with good penetration through.
Here's my trimming process for that same weld seam in the roof:
The lower corners of the opening were trimmed to size so that the roof patch could be clamped down into the drip rail. This will help to get more accurate markings on the roof skin for trimming the rest of the opening.
C-Clamp vise grips added to secure the overlapped panels for marking, then a straightedge used to read the crown of the roof to insure there were no dips or puckers along the top of the joint....
My favorite scribe, a local auction purchase...
Scribing the roof panel
Not much to spare! Trimmed the opening with some offset snips...
Corners touched up with a 1-1/2" drum sander