Dave, thanks for posting this. I too have a sandstone walkway and steps that I got the acid stains on. I was trying to wash some excess mortar off the steps and walkway using a pretty diluted mix of muriatic acid and water. A few days later nasty orange and yellow streaks and spots showed up where I believe the acid sat too long or didn't get washed off well.
After power washing, bleaching, and other methods I thought I was going to have the remove the yellowed stones. Today I purchased the oxalic acid in a crystal powder form from a local paint store. They are selling it as a deck brightener. I put some hot water in a bucket and dumped some oxalic acid in. I'd guess I put in about two to three cups of powder for about 2 gallons of water. I did not boil it, as it seemed to dissolve fine with the hot tap water I used. I did stir it up. I then poured it over the areas and worked it in with a stiff broom for about 10-15 minutes. I could see most of the stains dissolve about 10 minutes into it and some spots took longer. The water/oxalic acid mix would yellow as it lifted the rust/muriatic acid stains off. I then power washed the entire area really well. The yellow stained stones looked as good as new! I'm guessing like Dave posted that they may return and it will need another washing. I plan to seal the patio once I feel like it is resolved fully in order to prevent anything else like fertilizer from affecting it.
Thanks again for this post.