True, if you rub on a dry canopy. However, my experience using Plexus carefully for 25 years on my LS6's canopy has been that the canopy still looks perfect with no visible scratches.
Another benefit of something like Plexus is the reduced tendency to attract dust - important in desert conditions.
Finally, I think canopy covers are responsible for more damage than realized, when used outdoors in windy conditions. Sunlight doesn't hurt plexiglass unless a plane is outside all the time for a long time - which doesn't apply to gliders normally. A canopy cover beating against a canopy on a windy day is absolutely going to scratch that canopy. Much better to put the cover INSIDE the cockpit, covering the panel and upholstery, when leaving the glider out. For trainers tied out, a hard canopy cover that doesn't touch the canopy is the proper solution.
Where canopy covers are essential are inside a hangar, where they provide some protection against hangar rash.
I use a simple soft thick padded canopy cover for inside the trailer and hangar, and a thin reflective stretchy cover (that I put on after rigging and cleaning the canopy) to keep the cockpit (and pilot) cool and dust off until just before takeoff - when I can pull it off and stuff it behind me in the cockpit. Keeps all the grass and dirt blown back by towplanes from hitting my freshly cleaned canopy and some nice shade as long as possible. And always results in "you going to fly with that on?" comments from line boys...
Kirk
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