Your pool's liner provides a durable finish to the edges of your pool, preventing moisture damage to the concrete itself. Over time, due to exposure to the elements and various kinds of debris, your pool liner can become discolored and can even suffer from mold and mildew growth. To prevent this from happening, you should take care to properly clean and maintain your pool liner at regular intervals.
Cleaning a Pool Liner
For general dirt and minor stains to the liner, you can simply scrub it off using a pool-liner brush (available at most hardware and pool supply stores). These brushes are naturally soft and flexible, which means that they are unlikely to damage the liner—if you can't find a brush specialized for pool liners, you can always use a soft-bristled brush with a flexible handle instead.
If there is mold and mildew growth on your liner, or stains that won't go away with simple scrubbing, you may want to use bleach to kill the mold and remove the stains. However, you need to make sure that you don't apply the bleach directly onto the liner, since this can cause discoloration and even structural damage. Instead, make a mixture of one part bleach to four parts water, and use a soft rag to wipe or scrub away at the stain. Keep in mind that you'll want to do this above the waterline, or else drain the pool to scrub at the entire liner, as having bleach in your pool water can be a serious health concern and can cause all sorts of irritation to your eyes and skin. Once you're done scrubbing, be sure to wipe away at the area with a wet cloth to remove any residual bleach before refilling your pool.
Maintaining a Pool Liner
The best way to clean a pool liner is to make sure that it never becomes dirty in the first place. There are a number of things that you can do to ensure that your pool stays in relatively good condition and to reduce the overall amount of work that you have to do.
First, you should always use a pool cover when you're not using the pool for a few days. This will keep out organic debris, which can cause stains, and covers can also help prevent algae blooms by cutting off the sunlight supply.
Secondly, you should also check your pool's chemical levels at least once a week and add in necessary chemicals as needed to reach the ideal levels. This helps prevent moss, mildew, bacteria, and all sorts of other nasty things that can cause discoloration to your liner.
Finally, and perhaps most obviously, you should take a little bit of time to remove leaves, dirt, and other pieces of debris from your pool with a net. You should also make sure that you check and clean your filter to ensure that it is working properly throughout the swimming season—this can help cut down on staining and the amount of manual cleaning that you will have to do.