Proper care will add years to the useful life of clear plastics while improper care can shorten it to a single season. Glass windshields remain crystal clear with little or no attention, but the same is not true for plastic windows. Soft or hard, clear plastics are damaged by exposure, inevitably losing clarity over time. Care varies by the material. Soft plastics that can be rolled away are usually made from vinyl.
Here's some sage advice on cleaning your vinyl windows, This is based on an article originally published in US BOAT by Don Casey.
A walk down any dock will reveal that plenty of boat owners neglect or mistreat their clear vinyl (commonly called Eisenglass). Sunlight is the enemy. The reason clear vinyl turns opaque, then yellow, and ultimately brittle is the loss of plasticizers, most often due to evaporation caused by sun exposure. Just as with vinyl canvas, if you prevent the plasticizers from escaping, you extend the life of the vinyl indefinitely.
Two brands of clear vinyl, Strataglass and O'Sea, incorporate hard surface coatings for scratch resistance, which have the added advantage of sealing in the vinyl's plasticizers. All other clear vinyls, whether from a roll or a pressed and polished sheet, lack this barrier. The barrier that seals in the plasticizers in clear vinyl is a polymer coating, a liquid that you apply to both surfaces of the vinyl. Give this substance sunscreen characteristic and it becomes a two-for-one deal. It's best to start when the windows are new.
Even factory-coated vinyls need a periodic booster treatment. Strataglass specifies that you must use only Imar Strataglass products — 301 Protective Cleaner and 302 Protective Polish — to maintain the warranty. On other clear vinyls, you can use the protectant or plastic polish of your choice. Select one that includes sunscreen.
Before applying any treatment, wash the vinyl thoroughly. The imperative for gentle cleaning that applies to all clear plastic is especially critical for the softer surface of vinyl. Start with flooding to hydrate and soften dirt and salt. Paper towels are too harsh and will scratch the vinyl. Wash with soft cotton fabric — diapers, T-shirt material, cotton flannel, old terry cloth. Soap can extract plasticizers from the vinyl and remove previously applied seal coat, so use soap (never detergent!) only if you really need it. Pat-drying clean windows with a soft cloth minimizes spotting. A 90/10 solution of water and white vinegar can remove old water spots without damaging the vinyl, but rinse thoroughly.
Apply your treatment of choice to both sides of the vinyl, and renew the coating at least every four to six weeks. Reapply it anytime you wash the vinyl with soap. If you wash your windows daily, or even weekly, using an abrasive-free product will be kinder to the vinyl. Numerous manufacturers, suppliers, and fabricators recommend 303 Aerospace Protectant for its ease of application, slick finish, and effective UV screen.
While care takes a continuing effort, damage can happen in a second, so be vigilant about what chemicals come in contact with the vinyl. Never use glass cleaners or "all purpose" cleaners like Fantastic or Simple Green. Do not use ammonia, alcohol, acetone, or any petroleum-based solvents. Do not "protect" the vinyl with wax or with products containing petroleum distillates or silicones. When you waterproof the surrounding canvas, take care to protect the vinyl from the water proofer. Insect repellents are particularly damaging, so apply these far away and downwind of your vinyl. Many sunscreens contain chemicals that will fog clear vinyl. Be cautious not to handle or rub against the windows with repellent or sunscreen on your skin. The number of vinyl windows with handprints permanently etched into the plastic suggests that always washing your hands before handling the vinyl is a smart habit to form.