Second vote for SEM's bumper coat (and interior paint as well). I've used it tons of times when refinishing or repairing damage. If you can get the bumper PREP spray, do that too. It costs about $30/can (no kidding) but the bumpers WILL NOT PEEL if you use that stuff and clean them thoroughly before hand.

In my opinion, if normal, good-quality protectant isn't fixing the color of the bumpers, the rest of the chemicals are only stopgap measures before you either repaint or refinish it. Sometimes the finishes can be mottled or show streaks. When they fade or wear off, they can look strange too.
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1992 940 wagon, low miles
as well as others.