Back in 1985, I bought amt's Mach 1 and turned it into some kind of low-level racer using decals from different kits.
It was a real "quickie-project" - I don't think it took more than 10 or 12 hrs. - the interior and bottom got a silver spray paint using car rim silver (directly on the bare plastic - which works great), the yellow moulded body and wheels were simply clear-coated - that was it!
Pic of kit found on the internet: