we did try a single speed rotary but it wasnt toching it, what we found was the dirt was ingrained so no matter what chemical we used it wouldnt lift, funnily enough though you could use a sponge and it seemed to suck the dirt from the surface hence the idea of high pressure to clean it, the grout is that colour to begin with so chemicals wouldnt have made a difference to that, next time i do one similar i will try the chemicals you recomend and see if it works better as there is no harm in trying
