How do I feel about using it on cooking surfaces? Considering that oxalic acid is a major component of broccoli and other dark green leafies, doesn't bother me at all. If I stopped using BKF because of the oxalic acid I would have to stop eating all those great veggies too. And if the alternative is to use a non-stick teflon coating, with all the problems that can cause, it's a no-brainer for me. I use BKF on all my stainless cookware, and have never seen a residue from lack of rinsing. They wouldn't have that pretty, polished look if there was residue from the BKF. So, even if oxylic acid was dangerous, there's none left behind. You seem to have a thing about BKF, Linda. But have you ever read the labels on other kitchen cleaning products? Virtually all of them contain ingredents that are potentially harmful. For instance, Soft Scrub (which Friend Wife uses, so happens to be handy) contains "sufactants and hypochlorite bleach" and carries the warning: Avoid contact with eyes. May cause skin irritation." There's also a caution that prolonged use on certain surfaces can cause discoloring. Hmmmmm? Do we really want to use something that can discolor porcelain counters on our dishes? If we started listing the contraindications to using household bleach and household ammonia we'd be here the rest of the night. All in all, I think this is an issue in search of a controversy. But it's not a meaningful problem.

Be careful with these products. Whink is Hydrofluoric acid. It can cause severe burns and do not inhale any vapours. Wear a mask, goggles and gloves when using it. The suggestion of using a toilet bowl cleaner is a good one. I would try that first. Take the time to read the labels and check the MSDS before using any of these cleaners.

pressure washer, if that doesnt work, get a professional P washer out that has a heated wash system..... should take it right off... worst case take out the pavers in that little area, flip them, and re set

I believe (based on my own experience, not expertise) that it's just going to disappear soon enough. However, I just bought myself a steam cleaner which arrived today, and I have been happily cleaning just about anything I can find with it!! And I think I would try the steam cleaner on your concrete stains!