removing the dimpled effect is more effort than its worth as once you start sanding it, the plastic begins to fluff up in a way and it takes a while to get it proper smooth again, if anythin just buy pre-primed ones from seat or apply a shed load of primer to fill in the gaps and lightly wet-flat the strips to key it.
Or just lightly prime and paint, the finish will be still dimpled but it will be the right colour,
you dont need a spray
shop to do just the strips as you can rip em off and paint em with a can in your garage, its cheaper, quicker and you still get a decent finish.
the bumpers them self need painting properly, off the car job, can be done in a garage if you have the means (like myself) you could even do the bumper with a can, i did on my mk3 golf, looked good, took a day but it still looked good