polish before or after clear coat?

I'm new to modelling, so please forgive me if my questions are stupid. I'm building a 1/20 Tamiya Mclaren MP4/8 F1 car, and I've painted the white and red color scheme on the body parts. Do I fine sand and polish the parts before I paint the clear coat, or after, or both? I'm planning to airbrush Model Masters clear laquer over it. And what ratio do I thin the clear laquer to? And should I clear coat over the carbon-fiber decals I've put on the chassis parts? Thanks for any help you can offer.