After I thought it was finished I took to trying to clean the uncarved face a bit, also to remove pencil marks etc, but what I thought were surface marks went way deeper. This took me to think about sealing the surface again. Got some advice from my stone mason cousin (cheers Pete!) who tells me that putting milk on the surface causes a chemical reaction to the casein which seals up the pores in the rock surface.
He gave further tips that you can wipe the surface with potato before making any marks with say a pencil or something. It helps it to wash off later.
Coal or a water colour can be wiped on too to make more of a contrast with the rock when you cut it. Makes for a clearer distinction so hopefully more accurate strokes.
After ‘finishing’ this stone there have been another 2 sessions making a total of 11.
NB: The stone has to be totally dry before using this milk idea. The experiment above went horribly wrong for a while as a few days after the milk went on the stones blossomed in algae, lichen or mould or something. They looked like they had rotted! Took quite a bit of removing.