For what it's worth, I have had great success in protecting stone by using automotive 'clear-coat'.
All new cars are painted with a pigmented base coat and then top-coated with 'clear-coat'. This stuff is designed to take-the-weather and contains UV blockers to reduce the effects of solar radiation. I have used it primarily on counter tops made of stone (flagstone) and even some made of slate that was very soft (but colorful). It can be directly applied to the stone with no previous preparation other than to have it reasonably dry. I doubt this treatment would make the grade for the periods of time referred to in recent discussions - i.e. centuries - but it would help. Charlie Mead Rockwood, TN -
