There are three basic types of transparent plastics used on boats: clear vinyls, acrylics (the best known is Plexiglas), and polycarbonates (the marine standard is Lexan). Each has advantages and disadvantages, which is why most boats include some of each.
Vinyl is actually plasticized, stabilized polyvinyl chloride (PVC). It’s the plastic of choice for any window or clear screen that rolls, folds, stretches or needs to be sewn to fabrics. It is used most commonly in roll-up windscreens and windows because of these properties. It doesn’t have the clarity of glass, can distort vision, and will ripple in high heat.
Clear vinyl is the most unstable plastic commonly found on a boat. It has three distinct modes of failure: UV degradation from sunlight, which will cause it to become cloudy, muddy, and opaque; loss of plasticizer, which will cause it to become brittle; and mechanical scratching. A cleaner/protectant can help reduce plasticizer evaporation by coating the surface of the vinyl and can prevent scratching by keeping grit and salt from reaching the surface. Halting UV degradation is harder, but vinyl has blockers built-in, and a protective coating will help them do their job. Once vinyl degrades significantly, there’s not much you can do about it.
Vinyl pits, scratches and cuts easily. It’s soft and extremely vulnerable to grit and salt crystals.
In fact, one of the worst things you can do at the end of the season is to roll up your vinyl without cleaning and polishing it first. You’re likely to find it’s a scratched-up mess when you roll it out next year. Heavier gauges are especially prone to scratching. Heavily scratched vinyl is often beyond repair.
An abrasive scratch remover or polish will destroy the surface of vinyl. Use a soft cloth to clean it and a polish to protect it. Avoid citrus oil-based cleaners and also the relatively rare ones with alcohol in them. They’ll dissolve plasticizers, damaging the vinyl. Well-maintained clear vinyl can last five years and longer.
Acrylics and polycarbonates are varieties of hard plastics commonly used on porthole lenses, windscreens, and instrument covers. These plastics cost at least twice that of vinyl. Thick sheets can be ultra-hard, with enormous tensile strength and tremendous clarity, even better than glass. Moreover, polycarbonates have 100 times the impact resistance of glass, but weigh less than half. These properties make polycarbonates a good alternate choice for eyeglasses and airplane windshields. Don’t test it, but a half-inch-thick piece of Lexan should stop a bullet. Lexan is virtually shatterproof, while Plexiglas® acrylic will shatter. These plastics are far more resistant to UV degradation than vinyl.
Curiously, as hard as they are, their surfaces have pores that make them prone to catching and holding dirt, which can build up quickly. Polishes fill the pores, giving the surfaces a protective coating. When left unprotected, however, windscreens and porthole lenses can quickly become covered with an irritating network of tiny scratches. (A clear water rinse is wise after every outing.)
All too often, however, these scratches are actually the result of well-intentioned cleaning. A paper towel is just about the worst thing you can use for the task. Believe it or not, bulletproof plastic will scratch with a paper towel. Even worse than paper towels would be a hard brush or squeegee. Sponges often collect grit that can scratch. Use a clean, old cotton T-shirt or terry cloth. Throw them out or wash them afterwards because they can gather grit that will scratch the next surface you clean.
Ammonia, alcohol and solvents are destructive to Plexiglas. Don’t simply use your favorite household cleaner, especially on Plexiglas. (However, GE Plastics, which makes Lexan, recommends Formula 409 for simple clean-ups.)
Also, be very careful with any restoring compound you use. Some that we tested, like Mirror Glaze 17 Clear Plastic Cleaner, have more grit for scratch removal than you’d expect. Test any polish you’re using on a small, unobtrusive section first. In removing scratches, work the scratch remover in at a right angle to the scratches.