i have heard that a quick rough them up a little first helps a fair bit.
it should help take out any small bumps and help the paint to stick. the primer should fill any small sanding marks....
maybe give it a try on an old bumper? (if you have one) do half sanded and half not sanded. then you will be able to compare the two. a bit of extra work, but at least it will be right for the good ones/
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