If you have a stain that is pretty small - (less than 3 sq ft or so), you can certainly crush up some Vitamin C and put in a pair of pantyhose and rub the pantyhose on the stain, which should lift it. The stain issues I have been having with my FG pool is the entire pool, so the above won't work.
Currently, I've just done an AA treatment on Thursday and the pool is great again. But really having a hard time getting the pool to keep any Cl. I am trying Metal Free as the sequesterant currently with this AA treatment.
I did try the Extra Strength Metal Out from Doheny's and returned it (the unopened ones). I found that so far, the best sequesterant has been Metal Magic, which I order from PoolGeek.Com. By the way, not having much luck with Doheny's on the return and their customer service has not been that great.
I'm on a bit of a mission here - I am trying many different sequesterants to find out which one works best for me.
I also may try using Citric Acid instead of AA as Citric is much less expensive.
Finally, the advice given by many folks here does work. Since FG pools seem to be a pain with stains, keeping the Ph low and the Cl on the lower side helps. My CYA is 50 and I am running with 4.0 most of the time and a Ph of 7.0. Last year, I had a higher CYA and ran at least 7-8 w/virtually no stains. This year is a different story.