I want to go for this look

and me and my dad were going to smooth it when my car was going in for its respray but my dad suggested filling it with fiberglass filler and then normal filler but then said it would probably crack too easily and he simply didn't have the time to do it anyway as I was pushing it for a free respray as it was

I thought about plastic welding the rub strips but how would I go about this really? what section could I cut out of a bumper to plastic weld into the gaps? Anyone done this?
cheers