Buy a body saw. If you're working in tight spaces with many contours to manage get a cheap HF electric saw. The cord is easier to handle than the airline and the body is tiny and the blades are thin enough to make tight radii.

Use .023 wire. Even on 18ga your weld will lay down and greatly cut grinding time. The welds look nicer. You can turn down the heat and the wire speed and will blow fewer holes. It will also add less heat to the metal and the waiting time to cool will be shorter. You can use a damp sponge or a quick blast of air if you want but if you cool a weld too soon after welding the surface will shrink more than the backside and it will distort. Best, imo, to wait a bit and use air for cooling.

It is true, a copper plate will help cool a patch if the shape of the metal you're welding makes it practical. Flat-good, everything else-no go.

Always clean the replacement panel with a quick go over using a sanding disc before or after cutting the shape needed for repair.

A cheap electric cutoff tool works great to knock the tops off welds before you use a grinding or sanding disc to finish the seam. It adds less heat to the material and is smaller than a 4" grinding wheel. A worn 4" wheel is good for inside corners but a worn 3" cutoff wheel gets into smaller spaces.

I'm going to use 2K epoxy primer on any metal I make bare during a patch job, but maybe that's just me, and I learned to mix paint by density/weight rather than volume. That makes it easy to accurately mix small volumes so the waste is so little and the security of using a 2K is sweet.

Martureo is right about not grinding the weld down flat. He left it sticking up where it doesn't matter which makes the weld stronger. When I weld an exterior panel, if possible, I weld both sides for strength. It's often more than twice the work but the confidence in the product is multiplied.

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84 300SD
85 380SE
83 528e
95 318ic