As a quick insight for a quick job without too much hassle....after grinding down the arch face, lets say you chop out a piece the shape of a thin banana, and you are back to good metal. Get a sheet of perforated zinc and cut a shape a bit bigger than the hole. Tap the surrounding edges to slightly lower them. Screw a couple of screws into the zinc and fiddle it in behind the hole. Ask someone to hold the screws whilst you fill with body filler. Soon as it starts to set you can let go. Once hard, remove the screws, fill the holes. Once all set sand to shape, using a block. Heavy grit wet or dry, working down to a fine grit, about 1500. Then sand an area larger than the repair with 2500 grit. This flattens the lacquer ( just flatten, don't rub through it.) Keep well within this area when priming, and colouring, and lacquering. When all
thoroughly dry, sand down the whole area with 2500 or 3000 wet or dry. Then polish the whole area with some compound, clean off and then polish, and hopefully you should have a decent repair.
Wet or Dry should be used wet.
This is only a quick, brief info of the basic way to do it. I've had twenty years in this game, so do know other ways, but for a newbie to 'have a go' I hope this will help in some way. I did my Estate tailgate this way 12 months ago and it still looks good.
My amazement was not my job, but how exact was the aerosol spray supplied by Halfrauds !!!!!!! Nocturno blue. Surpassed themselves