Up to about 4 years ago I had a sports car with the usual Clear plastic "isinglass" rear window. On this car, pieces of window from an upholsterer friend and numerous customers and boat shows I became a bit on an expert on these windows. Actually this is clear polyvinylchloride (vinyl) without a fabric backing. It is made to be more flexible than most becuase of structure of polymer, amount and nature of plasticizer etc. The "natural" color of clear vinyl is actually slightly yellow and for this reason it has a slight blue tint and contains an "optical brightener" to correct the yellowness. Aging of the plastic. Four things happen, not always in the order below. All accelerated by air pollution, too much UV and misguided "care" procedures: 1 You will always see clear plastics of this type (even white vinyl that is similar except for white pigment) go yellowish with age. This is because the optical brightener becomes "worn out" after so many cycles of brightening in response to UV and then reverting. 2. The smooth shiny surface (as formed on rollers) allows for great transparency - close to that of glass. Then aging sets it. The first thing that happens is the formation of microscratches. This occurs because there is road grit on the surface and some people succumb to just wiping it off. Instead use hose of sponge and water and flood the area, then use a soft brush. Finally wipe it dry with nothing harsher than Kleenex or Cotton tee shirt material. If you had used our "Clear Horizons" first, the surface is more resistant to grit (more slippery) and if you use it after washing is fills in microscratches making the plastic more clear. You can use a VERY fine grit polish but to try to polish out the microscratches and silanize the surface but be aware that you are actually adding more scratches that can enhance oxidation and plasticizer migration (more below). 3. Like all vinyl, plasticizer starts to "sweat" to the surface, especially if the compresses surface has been scratched or "polished". You will see this first as a slight surface haze that is greyish in color. Almost any cleaner will remove this ( I use a little IGPink on Kleenex, then rinse and seal with CL Hor as above). You could also use some :Sea Glow" that replenishes lost plasticizer (actually making the vinyl more pliable) and also replenishing UV rotection and the anti-yellowing effect due to the fluorescent additive. Again rinse and wip off surface before sealing. 4. If neglected and/or cleaned with abrasives, the next stage (on the way to terminal) is surface yellowing. This is partly due to plasticizer on the surface drying out and changing (as on tops of cowl vents) and also oxidation of the vinyl surface (vinyl oxidation is yellow/brown). THe vinyl will also be yellowed and brittle. The latter process is accelerated by plasticizer loss, because that is what had much of the anti-oxidant additive. Like rust on steel or iron, the surface oxidation eats into the underlying material . While it can be removed by abrasives, it is best dissolved off. This cannot be done sagely with non-selective solvents such as acetone because the vinyl is also dissolved and the surface deterioaration accelerated. Our IG cleanser/conditioners (Sea Glow is best) dissolve off the old plasticizer and oxidtation that becomes like a sticky goo. The cleaning procedure is repeated (using paper twel soaked in product) until the stickiness stops coming forming and coming off. At this stage you are down to solid (with some breakdown plymer fragments). THe cleanser is neutralized with water and one can survey the good vinyl that is left. It looks like "frosted glass" because of the pints (just like rusty metal treated with rust removing chemicals). However the soaked in conditioner restores plasticity (brittleness is gone) and much of the yellowness in the vinyl material (due to the fluorescent addtive). Now IS the time to use an abrasive. Use rubbing compound, then metal polish to get rid of the worst of the roughness and then seal with Clear Horizons. We are looking for volumteers to also try MH SUperglaze that will fill small pits. Clarity will not be restored "like new" but you will be able to see (whtout fine detail) cars, pedestrians other boats etc) and the date for replacement much delayed. You will notice that the yellowing does not return for some time. Obviously , however it is best to start earlier in the process rather than wait until the end. There is one additional situation that we saw at a show given at a West Marine in Dayton Beach, FL. The boat had been moored near a paper mill and something (a chemical) in the air had "eaten" the dodger surface so that it was dull and cloudy. We used the above procedure and largely restored clarity. The photo was published inthe West Marine employee Newpaper. After a little feedback,I will post this, with some pictures etc in the newest (Vinyl) Trilogy. Incidentally, except for the brittleness a very similar process happens to the surface of lexan (polycoarbonate) , but NOT acrylic (plexiglass). The latter ages by a different process - "starring" or "crazing" especially around metal fittings. Thanks Jim W