The type of tool isn't as important as the grit or abrasiveness of the disk/stone being used although some will say that grinding can produce heat which can warp metal if not careful.
That said whether it's electric or air powered your going to be needing a grinder to take the weld down. Some will use grinding disks and others will use flap wheels. Some folks even using cutting disks to knock the bead down till it's almost flush then use one of the other means to finish off the metal.
Now some will say they don't grind the beads down till it's flush with the surrounding metal so as to not take off too much of the surrounding sheet metal.
There are welding wire products out there that are suppose to be easier to grind down "Twenty Gauge" and "Easy Grind" but the best way to avoid a lot of grinding is to not have a weld that needs it.
Your dad did show you right you'll just need to work on your technique.
Thoughts and comments expressed by me are mine based on my own experience and research and shared here freely. I am not a professional nor make any claim to be as such