Jack's Magic Iron Cobalt Spot Etching Stuff 900gThe Jack's Magic Iron Cobalt Spot Etching Stuff 900g (JMS01001) is a 900 gram specialized pool surface treatment for removing iron and cobalt stains from plaster and concrete pool surfaces using a targeted spot etching technique. Iron stains appear as rust brown discoloration; cobalt stains as dark blue black spots. The spot etching method treats the stained area directly without affecting the rest of the pool. Professional application recommended for