Ah - if its not painted; not sure how it will turn out by sanding the plastic.

But since its not painted; work away with 800 or more abrasive grit - thought it was painted and was afraid you might sand through the clear coat.
The E36 splitter is supposed to be a smooth shiny finish? So after sanding it might be smooth, but doubt it will be shiny, you might need a clear coat on it???