In reply to foxjerk:
> as polish increases, and with some rock types suffering more than others, does anyone think it might ever be acceptable to either wire brush the offending areas, or even apply a dab of acid to regain some friction?!
>
The polish has happened through the passage of thousands of feet, flattening the original texture. Adding acid or wire brushing will only make the problem worse, there may be a temporary improvement, but then it will polish up even better than before.
On the Continent the only 'working' solution I have come across is the addition of a thin smear of 'sika' which is harder than the rock, less inclined to polish and adds a bit of texture.
Chris