Looking for suggestions on how to clean Granbury stone and Oklahoma flagstone where areas of standing water have caused the mortar to get a black coloration to them. Can you power wash this sort of thing or is that too harsh? Is there a chemical that can be used to restore the natural color of the mortar? Thanks in advance!

I used a pressure washer with lower PSI when I cleaned our flagstone patio. I think it was only around 1200PSI. Worked well and no damage.

bleach / water mixture scrub, let it sit for half hour, then power wash at lower psi