We've had a fiberglass pool that had a large hole in it for several years. It set 1/2 filled with dirty water. We pulled the pool out of the ground & repaired the hole; however, there is a dirty ring where the water set. We've gotten it to fade with scrubbing, but haven't totally gotten rid of it. We need to re-gelcoat the repaired area. I'm afraid that the best way to get rid of the stained are is to re-gelcoat the whole pool. Is there a certain brand of gelcoat that anyone recommends? Will we have to sand the whole pool to get it to adhere or will we be able to just roll it over the old since it's still adhered good?