As to that battery acid stain . . . had a customer where red dirt had been tracked onto their concrete lanai porch so long that during the curing process, the surface became dull red - the dirt actually became a part of the aggregate (this happened over a 20 year period). No amount of PW would clean it off. I finally used phosphoric acid and a scrub brush to etch the surface off. Took a lot of scrubbing. Got rid of the red and looked good afterwards but the drawback is that now, the surface is not so smooth as it once was as the sand aggregate is now somewhat exposed. If you want a non-skid surface, then it's ok. Probably should do a whole section out to the nearest expansion joints, though. Phosphoric acid is available at painting stores and OBTW, this stuff is a good rust remover, too. Use it to eat rust off of car parts - just throw them in a jar full of the stuff for a couple a days.