Say what you will about painting enamel on rock art (and I think it looks bloody awful) I suppose it would at least protect the physical materium of the stone from the ravages of the elements. At least for a century or three. But it would mean that eventually the surronding stone would erode down and when the paint was eventually eroded away, there would be nowt left to indicate a motif had ever been there.

I don't think the issue should be about how clearly 'enhancement' shows up the motifs, the pre-eminent consideration ought to be its potential for damage to the rock art.