Hi Celeste! To prepare my shields for customising I use a sharp curved blade from
on an X-Acto craft knife, then I use first a 180 grit sandpaper, and then a 400-grit waterproof sandpaper with water. The part will lose its shine so you can either go for another sandpaper 1000-grit waterproof) or an aceylic varnish.
Hope this will help.
Greetings from Paris,
Arnaud