My masonry heater installer gave unclear instructions, and first time I used heater (10yrs ago) - water came out and stained water lines down the exterior. I also have slight soot discoloration over my main door and pizza-door. How can I clean this - without rubbing the soot 'into' the stone? It is simple uncoated tulikivi stone. (Was wildly expensive - and after 10years of hating the installer+buy experience - a supplier active in this forum - i am going to fix these stains if feasible).