have to do this crap at work all the time.. Def the best way is to prep it with 180 grit then high build prime it. block it and prime again if you can still see the texture. NO need to SAND ALL the texture off before paint.
Yeah I sanded almost all of the texture down and then used a high fill primer and blocked it out then primed again and sprayed some color just to see how wavy it was, I guess that was wong and I can just keep priming and blocking?
keep in mind that its a flexible abs plastic.. and you will never get it completely straight. once painted and put back on the bumper you wont notice alittle wavy in it.
just make sure when you paint it. if you dont paint and clear it when its on the truck (what we did) be very careful or you will crack the paint if you have to form it back at all. but na yours wasnt "wrong" just a little bit more work then you need'd to do. im guessing you wont have that line in it anymore?