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.