If the ding isn't terrible, you may look into having a paintless dent repair person fix it before you paint. That would save you some material cost and time.

Otherwise, you will spot prime the repair area(s) so that it fills the scratches left in the plastic filler. I typically finish plastic to 180. Finish out the primer to 320 dry like the rest of the car, and epoxy prime. That will do several things; act as a uniform basecoat before the topcoat, tie all the coats together chemically, bite into the 320 scratches so that the paint doesn't delam like before. Nice thing is most epoxy primers stay "open" from 3-7 days so you can get the topcoat on at your leisure or as weather permits. If you wait beyond that window, you have to wet sand and re-epoxy.