On Sep 10, 3:42*pm, The Natural Philosopher
wrote:

sandstone is not damaged by acid.

The OP has observed otherwise, and Cormaic's pages at pavingexpert.com
also describe the same problem. The various metal salts in the
sandstone can react with HCl to produce a different colour.

Note: I am pretty sure the OP is describing immediate staining, not a
lichen buildup.