This is only from my experience, I do not have a chemistry degree. One thing I noticed is when the pool gets milky white after the ascorbic acid treatment the stains don't tend to come back. I think it is when the calcium binds with the HEDP and then it turns the pool milky. It does filter out eventually, which is why I think that the stains don't come back - the metals are filtered out with the mess. I know that I have had to use a lot of ascorbic acid this year, and I never had the milky mess. I also did not have my calcium any higher than 250, so maybe I did not saturate the water enough, and my calcium didn't drop much. Other years my calcium dropped about 150ppms. I don't know which is worse, or more expensive. I usually use a floccing agent when I get the milky water (after about a day). So that is expensive, the calcium is expensive, and the HEDP is expensive. It is your call, but if the stains are only on the steps I would just put some ascorbic acid in a sock and rub it on the steps. I am just wondering if it is only on the steps, or maybe you don't see the stains on the liner. Is your water blue, or does it have a greenish tint to it? I have a white fiberglass pool, and when I have staining it turns the pool yellow, so blue and yellow make green - which gives my water a slight green tint. It isn't very noticeable, but since I am a stain nazi I can pick it out right away.