I use a thirty year old exnavy 180 rpm rotary with a scrub pad for real dirt and finish with micrfibre onsite recyclable pads. Fusion Clean prespray. I used to do an office on top of New Sreet Station that was used by the railtrack engineers, with all the muck they brought in. Over 100 mt per hour and a good job done. Only drawback was the terrible access, it took us 3/4 hour to get the gear in.
Being semi retired I have stopped commercial work. 6 o'clock on a Sat morning no longer appeals.
Pads around £20 each, need 2 or 4 rinse as you work. Solution worked out around £10 per job.