http://www.flippyscatpage.com/carpaint.html
The quick answer is that car duco - if its in half decent condition - is extremely hard (its baked for at least half an hour at over 300º since it has to withstand road gravel hitting it etc). And cats claws are the same hardness as our fingernails. So unless you can scratch off the paint with your fingernail, there is *no way* a cat can scratch the *paint* of the surface of a car that's in a fairly good condition. (Paint does weather though and if its in a really bad state, it can be scratched, but if its that bad it should be a funny colour of white and be in the junk yard).
However, if the car has been *waxed*, the cat's claws (and your fingernails) can remove some of the wax, leaving what looks likes scratches in the paint, but are in fact just places where the surface of the wax has been removed. But then again a cat just walking across a car doesn't *use* claws, so the only way that the cat could scratch the car is if it was trying to somehow get traction on the surface, by either falling off or (dare I say) running for its life.
The worst a cat can do to in normal circumstances is leave cute little muddy cat prints - annoying but not inherently damaging.
This article is Copyright © Victoria Chapman, BSc, Paint Technologist,
and printed with permission of the author.
