Most good-quality vinyl shower curtain liners will survive years of washing. This method works very well with top load washers.
Our first attempt to do this in a front-load type did not turn out well. We did not succeed in removing all of the dark spots. A top-load washer allows the liner to sit in a tub full of bleach solution. The front load one does not. We will try several more times and report back when we have found a successful solution.
Here’s how to do it.
Fill your top load washer half-full with hot water. Add half the normal amount of detergent that you would use for a full load. Add half a cup of bleach.
After agitating the load for a minute, shut off the washer. The liner may again try to float to the top. Push the vinyl liner and the towels back down into the water but don’t use your hands. We use the rounded end of an oversize paint-stir stick.
Let the load soak for 30 minutes. Then start the washer and run it for a full but short cycle. Set rinse on warm, not cold, so the vinyl will be somewhat pliable when taken from the washer.
Even after the spin cycle, some water will remain in the folds of the liner. Use one of the towels to catch the water drips as you carry the liner bundle to the tub. If the liner is still stiff, run hot water over it before placing it back on the rings.
Turn on the bath fan, pull up the fabric shower curtain in front, if there is one, and let the liner dry out for a few hours. The remaining wrinkles will come out with the next hot shower.
By the way, the easiest-to-use rings are the double-hook type, since you can remove the curtain or the liner as needed.