My suggestion for you is to put the DA away. Those are time savers at best but self defeating for the beginner. The DA is fine for knocking down heavy plastic but dangerous for skim coats. When trying to get a smoothe pre painted surface use 320 wet paper wrapped around a wooden mixing stick. Use even pressure and lots of water. Sand diagonally and don't concentrate on one area. Use a straightedge as a detection guage to point out obvious lows or dimples. As you are sanding the area, run the straightedge ( try a metal paint guage ) on its edge along the sanded area. If you sight along the area of the straightedge that contacts the flat surface, you will see air gaps where the dimples and lows are. Once you think you have the surface error free, spray a scratch coat ( non lacquer ) of some odd ball color on the area. Let it dry and then lightly wet sand with 320 , again using the wooden stir stick as a paddle sander, removing much of the scratch coat. The residue that is left will point out obvious lows and dimples. At this point, you can use a heavy build primer on those areas. Layer it on the affected area, with lots of setup time between layers.Omni makes a really good 2 part build primer that is inexpensive, dries quick and builds areas big time. This can be used like liquid filler. If you have a jamb gun, this works great. It is like air brushing. I have filled dings and lows as deep as 1/16th inch using this technique. After drying, wet sand again using the stick and 320 wet paper technique. The imperfections will eventually disappear. Lots of work, but that is why big buck paint jobs are big bucks. Patience and perseverance. Make sure you use 320 paper for wet sanding. It will hatch the surface adequately for adhesion. Remember that Evercoat or similar products have a shrinkage factor and will not hide dimples and dings completely. The build primer will. Good luck pilgrim.
P.S. If you use a sealer coat before you finish paint the car, you will see what the job will look like before you paint it. Try a PPG DP product.Once you seal the car, what you see is what you get. Keep in mind sealer should not be sanded, before final paint.
Walt in SoFlo