Hi guys, just wondering what your thoughts are on whether or not I should sand smooth (and if so, what grit?) my Kawasaki Ninja's wing mirror before making a mould for it? It's made from a cheap-looking, rubberised type plastic with a rough (but uniform) matte surface (a bit like a car's bumper but slightly rougher). Thanks!

Yes definitely. Sand it smooth with 400 grit paper, then make sure you use PVA release agent. Spray application is better if you can.