PVC stands for polyvinyl chloride, these are lightweight, relatively flexible, and sealed together with specialty tape. This is the most common form of pond liner that you'll find in most backyard set-ups. PVC is thin, and often looks like large, flat pieces of garbage bag. Sunlight degrades PVC incredibly fast, and any liner left exposed to UV radiation will quickly become brittle and ineffective.
Once PVC begins to degrade, repairs become increasingly difficult. Usually, a damaged PVC liner needs to be completely replaced.
