This is a residential condominium basement concrete floor, only about 500 square feet. You can see all the drywall mud and paint spatter and overspray left from the building process.
The concrete itself, though, was in great shape for staining, and we didn't want to damage it during the cleaning process. So, we wet-scrubbed the floor with a rotary floor machine and black stripping pad, scraped problem spots with a 4" razor edge hand tool, then dry-sanded off any remaining residue..
With the floor perfectly clean, we spray applied acid stain, in the color of "Sunset". You can see the "mud-cracking" in the concrete caused by inadequate curing at the time of the original pour. That cracking actually enhances the effect of the acid stain, resulting in a more natural "stone" or "marble" appearance.
We sealed the floor with two coats of a clear, water-based epoxy. The sealer brings out the true color of the acid stains, and creates a wet look.
This floor ended up with a perfect staining result - despite the original mess. Most contractors, though, wouldn't even have tried to get this floor clean. They would have just recommended grinding everything off and applying metallic epoxy. Not only would this have cost the customer more, but it wouldn't have achieved the same natural-looking stone result.