Sorry, but I've been taught different. The idea behind getting body filler straight so that you can't see any sign of it with glossy paint on it is that you have to cut the filler with very heavy grit sandpaper, or better yet, a cheese grater while it's still in plastic consistency. The cheese grater gives a rough finish, but it cuts so well and gets your panel straight. I've used it for under a quarter inch of hard-to-sand fiberglass impregnated bondo.
You will have a rough finished first coat of body filler, but then again, it is aligned with the rest of the metal panel. Then you go over it again and fill in the roughness. Hit it with 40 grit but don't take it quite all the way down. (You see, light sandpaper won't cut through the filler to shape it flat). Then take it down further with 80 grit and go to the metal again, like you did with the cheese grater. You'll have most of the roughness out of it.
You'll still see this if you stop here. So you want to get glazing putty (preferrably NOT lacquer based) and spread it on top. Then hit it with lighter sandpaper like 100 or 120 and feather-edge it.
Finally, spray some primer, let it dry, and you'll probably see if you did a good enough job or not. Then very lightly spray a different color primer over it...just dust it. Then block sand it and you'll see any low areas then.
With the modern primers, you can put filler over them, but at least scuff the surface first. Preferrably, filler sticks best without primer, but when detailing like this, sanding out the primer could cause more problems. The filler I'd use here would again be the glazing putty.
Then primer it again, and guide coat it again, until you get it imperceptible to sight or touch.
It is best if you keep your filler as thin as possible. So try to work your metal to where you can keep an 1/8th inch or so of filler at the most. You'd probably be okay with a quarter inch in places, but try to get your sheetmetal close.
This is an art, and it takes a lot of practice, so don't get discouraged.
