In reply to aln:
If that rock's the volcanic basalt/doleritic stuff it looks like from the photos on the logbook page it may just wear wire brushes out! I really don't believe wire brushing significantly damages this stuff, unless you overdo it and the brush metal rubs off onto the rock and polishes it a bit.
However, having said that the large areas of white lichen in the shots look like crustose lichens, which are ingrained into the rock. You can give the walls a brush over and get the thick off, and you should be able to get holds clear of them by brushing, but the lichen'll come back. I've never had much success. Crag Lough is covered in a rich collection of these lichens, they eventually disappear off the holds by usage over the years, but the walls are covered in them.