I removed, prepped and painted a diffuser a couple of years ago, not a Golf but doesn't matter in this case. I actually used Hamerite Black from an aerosol, a few light coats did the trick. My thinking was that whatever etches onto metal in this paint is probably good enough to etch on to plastic.
Not had any chips afterwards, in the three years until I sold the car.
Was quite impressed with my workman ship, and as the diffuser is low down it doesn't have to be as good as the trim nearer to the eye.