400 grit is too harsh, its a starting point not finishing. Either buy a headlight restoration kit (
like this) or wet/dry with soapy water starting with 400 and working your way through progressively finer grades, 800-1200-2400 then a cutting polish, soak the wet/dry in the water before using it.
As you've removed a layer of the plastic its a good idea to finish off with a spray of UV resistant lacquer.