They do make some brushes that are 50/50 SS and polyester. You can also get stones that will scrub some of the top layer off, but be careful not to make the step too rough, takes good bit of time and pressure to screw it up, you will know it if you just scrub and check scrub and check.

This brush did not work for my stains, what I did was pour some Muriatic Acid out just over the stains, and the acid burned the top layer of the plaster off.

Just in case you are not familiar with working with Acid, you must wear goggles/sunglasses and rubber gloves help if you are not found of the burn of acid on bare skin.

We had some persistent stains and I used a 1.5" PVC pipe covering the stain, poured a little acid in the pipe and presto! It burned the stains right out, just a FYI only takes like 30-60 seconds to burn off the top layer of plaster. Add a little wait and check, if that does not work add a little more. For the steps you can pour straight on the steps, takes a little more Acid but will work well. I have done this 3-4 times when the kids leave something metallic in the pool when I am not home and it works like a charm.