[happy1]

Someone may have used acrylic paint instead of 2-pack system. Is the paint flaking off?

I use an electric sander with 180 grit If I want to remove paint cracks on a bumper. (dry).
If you go through to the plastic you will need to use plastic adhesive chemical on those spots. You will need a 2-pack primer/filler.

You can also use more course sandpaper such as 80 on the sander if you are removing scratches and need to go a little deeper in certain areas. I would then use a 180 to make it smother before applying the primer.

You can use 2-pack filler / putty to fill the scratches if they are deep. The sanding machine will sand away excess filler / putty.

When the primer has dried you need to wet sand with 400 grit and ideally 600 towards the end, before applying color base and clear.