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The simple act of applying a coat of paint to the rock would act to retard the weathering in a pronounced way, no one should even attempt to argue with you on that point. If they do then they can go check out any old wooden structure with, and without paint.
Also when the structure was created initially it was of the same aging point in time, but since the re-structuring of the head the head is of a "newer" era, regardless of the quality of the stone. This would be like replacing part of a 50 year old wooden fence with a new section. Gee, I wonder if they would look the same quality?
The fact that the head is of a different quality(hardness) of strata makes the argument of "quality difference" even more preposterous.