If I'm building a car to be displayed alone, it is done up in pristene condition. If I'm using cars in a diorama, however, I think a few "rubber donuts" look pretty good down the side of one of the stock cars, or mud and wiper cleaned filthy windows on a 4X4... or made blazing through some snow covered field!
Rust around the fenders of an old tow truck in a diorama, and a glassy sheen on the centerpiece of my mantle. My centerpiece happens to be a 1971 Chevy Monte Carlo made up like Bobby Allison's "Coca-Cola Chevy". It's got a super slick finish, with the decals looking like they're part of the finish itself, but I blurred the raised white lettering on the tires, as well as blurred the spaces between the holes in the wheels, so the tire appears to be in motion. It's mounted in a 1/25 size display case, as if he's in a turn at Bristol by building a mock track surface and retaining wall (that also happens to be blurred). It's amazing what kind of realistic effect that has! But that could be considered a diorama I guess.
"Yep, then they are weathered!"
Frank