We applied thinset white mortar on the brick of our house using the German schmear technique. After a very very hard rain, we had some rust stains show up down the brick in areas around older gutter downspouts and our chimney. We are replacing those areas with new downspouts and painting over the chimney cap with a rust-preventive paint to try and prevent this from happening again. Is there a way to seal mortar to prevent discoloration or rust stains from sticking to it in addition to our replacement of parts? We have tried removing the stains, but decided to just go over it with another layer of thinset mortar because it still showed stains after treatments. All advice appreciated.