Spray it down with an industrial alkaline cleaner and then use a surfacing machine, attached to a 20hp 3500psi 5.5gpm pressure washer that has a boiler set to 200 degrees, and recovery equipment to **** up the mess. The surfacing machine works like a lawnmower, but the blade is a pipe that has spray nozzles on it.
That is the best way, without using something abrasive. It is pretty quick and much easier than scrubbing by hand. I used to do it as a pro. We cleaned concrete, tile, and buildings for Dodger stadium, la staples center, Nokia theater, many restaurants and also cleaned floors for pro epoxy guys. We did a garage that was used to tear down engines for rebuilds and it really made a huge difference. The cleaner would etch anodized aluminum within a couple minutes and it would burn your skin in a couple minutes as well. Once diluted, it was safe for food areas. Find a company that does the same and get a quote, if you want it done well and easily. The price may not be bad for how well it turns out.
You could rent a pressure washer, but a wand is very slow, and most pumps are not made to run hot water. Our pumps pushed cold water into the boiler, so the pump was always cool, unless they were left on bypass for too long and they heated up the 300 gallon tanks that they ran from. A floor scrubber would be better than scrubbing with a brush.
My dad and I scrubbed a 2 car garage by hand, but it was not so bad that the grease was caked on. It was pretty dang dirty, and we did a good enough job that the rustoleum epoxy is still sticking to the concrete.
