3M and SEM have an aerosol clear (it comes in colors too) called chip guard in varying degrees of texture. It's used mostly for rocker panels, but I'm pretty sure if it'll stand up to rocks and gravel, it'll take the soles of shoes.

[b]1957 Chevy Big Window 3100[.b]

In the Stovebolt Gallery
More pix in Picasa

It ain't broke 'til you can't fix it any more.