Perhaps the most common liner problem is discoloration. 

It can be traced to a number of sources, according to APSP’s Basic Pool & Spa Technology: overchlorination, the sun’s UV rays, chemicals applied directly on the surface without proper dilution — even stains from a wooden poolside wall.

For aesthetic reasons, a pool owner may want the stain to be covered up, and a patch might be just the thing. However, if you’re dealing with a faded area, the liner is more likely to tear and eventually will have to be replaced.

At least stains on pool bottoms caused by fungus growing in the material below the pool are a thing of the past. All vinyl used for inground pool liners now contains a fungicide additive.