How do you get rust spots off the bottom of a Mercite pool?

Get 3"dia PVC.pipe cut to lenth,Invest in a small fountain pump there about $25 buy a wax ring that plumbers use for toilets.attach wax ring to PVC.pipe and put over stain you will need to put step latter in pool and hold pVC down. insert pump and pump water out of pipe.remove pump and hold hose to keep water from draining back down pipe. pour in myriatic acid and let set 30 min. take a soft brush or rag and wipe clean