>>10228170800+ is ALMOST pointless for parts that are going to be painted unless you somehow don't have access to leveling paint.
I don't know what you mean by "super scuffed" but maybe you're just using sandpaper that is not fit for this hobby that leaves deep scars even at 600 grit, chances are that you are just underestimating the leveling properties of good paint as you haven't even tried to apply primer yet.
Here's a helpful product for you