If the Atlas Grey is a light grey, get a white primer, along with some 1200/1500 grit wet & dry.
If the dent is below the swage line in the door, this job should be a doddle. Remove the bump strip and place a piece of masking tape along the part you DON'T want to paint, as this will give you a 'datum' line in which you can work from.
If you intend on going all the way down to bare metal on all of the door, start rubbing it down with 60 grit, but don't apply loads of pressure as you'll put deep scratches in the metal, and you'll be putting more filler in than you first thought.
Once you've got it back to the metal remove all the grease and dirt with some thinners making sure you don't touch it with bare hands as the grease in you skin will react with the primer.
Start applying the primer, dont worry if you get runs in the primer as it's a piece of cake to remove them. Apply about 3-4 coats of primer and once it's dry start to flatten it back with 1200/1500 grit and water. Also don't worry if the paint reacts with the prepared surface, it's only primer, and once you've got enough coats on it'll flatten back easily.
Once this is flat, then prepare the surface again with thinners, and wait to dry off, and then comes the best bit - Top Coat!
Apply about 4-5 caots again, but remember not to go over the top with the first coat, just give it a light once-over, as the consecutive coats will cover up any slight blemishes.
Once you've got the last coat on there, leave it for 24 hours, unless you've got access to some heat lamps.
Next step is to flat the top coat with some fresh 1200/1500, again don't go too mad as you'll go down too far exposing the primer, this is just to make the surface completely even, otherwise when you apply the lacquer, it'll look uneven.
Now comes the fun part, it's now time to polish out the marks left by the wet & dry with something like t-cut, or Autoglyms super resin polish, as it has abrasive particles helping to get rid of marks.
When it comes to the lacquer prep like before, and again apply about 3-4 coats, and if you get any runs again do as before, rub it back (once dry) and polish back up.
Phew........a bit long, but Hope that helps
Lewy