Sheetmetal work in itself isn't terribly difficult, it just usually is, because you have to work with metal that is paper-thin and riddled with little rust holes. (When you're lucky that is.)
What you see here, is a lovingly mouthbitten hole, that allowed for a pass-through of the trailer hook's wiring loom. It is a bit hard to make out in the pictures, but it has been an eyesore for me and that says a thing or two about it, considering the initial state the old Comrade Car was in, when I first got it.
Patching holes, as I mentioned before, isn't terribly difficult. Depending on the size there are two options to tackle them. Number one is to cut a patch from some sheetmetal and weld it in. (No big surprises here, I suppose.) Number two is to use a non-welding metal, e.g. thick copper plate and use that as a backing and heat sink and use your welder like a hot glue gun.
You start at the edges just with some small spots and work round them in a spiral until you hit the center. Then you turn down the feed and crank up the voltage, still with the backing plate in place and give it some heat to close all the small pin holes, between the spots of weld you just created. This will leave you with this somewhat ungainly blob of weld.
This is the moment, when a diegrinder with a carbide burr can really shine. Depending on the spot different burrs might be the optimum tool. Here it was a cone-shaped, round tipped one, which I used to flatten the weld. No need to be perfect, but to remove the bulk of the metal.
Final step is to use a 50mm flap wheel on a 6mm shank to blend the remains of the weld into the surrounding metal. It's still visible when unpainted, but you can hardly feel the ridge.
There you go, bit of primer thrown on top of it and the job's done. (Don't forget the backside.)
Now if one wanted to do it perfectly this would be the moment to give it a light skim of filler and sand it down flush and it would be absolutely invisible. As the comrade will wear a rolled-on coat of paint and this corner will mostly be hidden by the number plate, I have to admit that I won't bother with filler in this one spot.