I got my Gialla lip painted about a month ago and the guy who had painted it just threw the paint over it without any prep. It looked perfect but it didn't last even a week. Its been peeling like crazy and half of the paint is gone now. It didn't stick well to the poly at all.
I talked with the guy and he said he would re-do it but I have to do the prep and he just sprays it over.
I was thinking maybe hitting it with 600 grit sand paper, then throwing on a couple coats of primer, should that be enough?

 

With urethane you have to do it right. We always recommend using an organic cleaner and a Scotch pad to get any of the mold release agents off the urethane. Prior to that, let it sit in direct sunlight for a while to heat the part but and let the release agents "sweat" out.

Regarding priming, if the bumper is new I first use Bulldog brand
sanding soap to prep them, followed by Bulldog brand adhesion
promoter and then several coats of PPG epoxy. It states in
their information sheet to use the slower acting catalyst on
urethane parts. I then lightly water sand any orange peel, spray
another coat of epoxy mixed as a sealer, followed by
color and clear. I have been using their flex additive
in the clear, as I have been painting them off the car.

On used bumpers where some filling is needed I use several coats
of PPG urethane primer and water sand with 600 paper. Follow
with one coat of epoxy mixed as a sealer and then spray color
and clear.

I do prefer to paint them off the car as I can paint all the edges of
the bumper and the edges of the car. If I paint them on the car
I loosen them so a gap is between the bumper and the car to enable
painting the edges. This also prevents the paint from filling in the
gap between the car and bumper.

I am not a professional but all the bumpers refinished this way have held
up very well.