Don't know if you feel like backtracking at this stage, but maybe on your next project try this.
Fill and sand SOP. Give the filler a top coat of superglue as a sealer. If it looks like the filler "soaked up" the superglue, give it another light coat.
Lightly sand, feather and polish the filler/superglue into the plastic.
I've had good luck with this method, with the filler areas being indistinguishable from the plastic under the primer, regardless of the type or color of the filler.
TKB