Welding Tips and Tricks

So being new to the world of metal, I would like to find out more about the "finishing" process of grinding the welds smooth so the piece can be painted. Are the processes the same for all metals or do you use different techniques for different types of metal?
Just wondering.

If it must be ground, thick or jumbo flap disc is the only way to go. 40 grit is all I usually use, lighten up on the pressure to buff out the scratches. Also, unlike a hard wheel, keep it as flat as you can to make use of the entire pad and not just the outer corner.

I have only done aluminum and used a grinder with a disc 60grit and a d/a 6"with 80grit and Boelube on both and then a heavy-duty 6" sander with 100 grit lubed as well. This was the result:

The last is what I started with and the first was the end result.

Ok, so today I was doing the opposite end of the spectrum and found that there are different methods. Was cleaning up corner welds on stainless steel and used different grits and no lube. Also used the tool to finish the "grain" of the steel (which I thought was pretty cool). With the different jobs we have going on, I should be a fair hand at this before too long. Since I am not able to fab yet, I'm a helper of sorts and use the experience to learn about the other parts of the equation.

If GMAW is the process (aka MIG) , use as little CO2 as possible since it makes welds hard and difficult to grind. Wire meant for easy grind is available but it has limited strength compared to ER 70 S6.
On sheet metal, watch grinding heat as it will warp. But this heat can be used to shrink metal with quenching. It is a skill that is high art in autobody repair.