A decent paint shop would squirt it with a suitable primer so that the fresh paint will bond to the original finish.
When sanding down the old finish, a few drops of dish detergent in the water you're using helps a lot.
Final sanding of the old finish before laying on the primer should be made with no higher than 400 grit paper. The same thing goes for laying down the colour coat on top of the primer.
If you get sand happy and dig down to plastic, you'd be best to prime those areas with a special primer. A bodyshop supply store will be able to point you towards the right product.
Don't forget that the flexible front and rear bumpers need to have flex agent added to the paint.
Nothing else obvious is coming to mind at the moment.
