I haven't had very good luck getting rid of cobalt stains permanently. That may be because I'm not keeping up with the doses of Jack's Magic like I should.
Cobalt stains appear as raised uneven deposits usually on the floor of the pool. You can get rid of them by spot acid washing or by using a pumice stone. I recommend spot acid washing. The way to do a spot acid wash is to take a piece of pvc pipe that's long enough to reach the cobalt spot and place the end of the pipe on the spot. Put a rubber glove on the hand that you'll be using to hold the other end of the pipe.
Pour muriatic acid in the pipe in small doses. After a minute you'll see the acid coming out of the other end of the pipe and bubbling on the cobalt deposit. Keep adding acid until the cobalt is gone. It shouldn't take much. Now the cobalt is in solution in the pool. To keep it from redepositing on the pool finish add one or two bottles of Jack's Magic The Pink Stuff according to the directions. If your pool has a salt chlorinator, use The Purple Stuff instead of The Pink Stuff. This should keep the cobalt in suspension and the filter should filter it out of the pool. Like I said in the beginning, you have to maintain a certain level of Jack's Magic to keep the stain from reoccurring. There is a test kit for Jack's Magic. I just add a quart every other month to a normal size pool.
