Well its not unless you have stains vs dirt on the surface.. Stains penetrate the substrate, therefore to remove them the only way is to remove the substrate. To remove 1/32 or 1/8 of an inch is not noticeable. My plaster is a minimum of 3/4 inch thick and in most places 1" + so the 1/8 removal is nothing. This was per the plaster company. They also do an acid wash on start up with my product (Diamond Brite) I have seen them do the acid trick on several pools. The amount of time you leave the acid active (concentrated in the pipe) is the key. I also used Acid on my plaster pool and coping years ago to remove rust stains, worked fine.

Try the brush first, if it works great, if it is stubborn dirt it may work. If it is a stain it will probably not work. You are doing the same thing with the brush, rock or acid. The speed at which you remove the substrate is the difference. You can always hire a company to do the process if you are uncomfortable. It would sure be easy to pour a little acid on the step first to see the results.