Bumper scratch repair

[Catfish]

I'm looking for advice on what product to use to repair deep scratches - caused by a screwdriver or similar weapon - in painted plastic bumper.

The scratches are less than .2mm deep, but too deep to hide with touch up paint and too shallow I think for conventional body filler. I want to fill the scratches with something tough and then sand back smooth with fine paper before respraying.

Any advice welcome. Thanks.