Many of our motorcycle windshields are made of Lexan. You might also find motorcycle helmets with shields made of Lexan. Some goggles are also made of Lexan. Lexan is a wonderful material, stronger than glass, and much lighter. The problem is that Lexan will easily scratch if you don’t know how to clean it and what to avoid. When a manufacturer says their item made of Lexan is scratch resistant, they are not saying scratch-proof, so I guess I shouldn’t accuse them of lying. It does make me wonder. Bikers USA had a face shield to experiment on, so I tried lots of products found around the house and in the garage. There are some products made specifically for cleaning Lexan, but the prices are way too high (as much as a dollar per ounce!). A good substitute needed to be found. Stay away from Windex, Pledge, Isoproyl Alcohol, and anything alcohol based. Stay away form eyeglass cleaner. Lexan is much different than glass.
Stay away from soaps used for washing cages.
At the gas station, avoid the temptation to use whatever formulas they have for cleaning windshields. Lots of plain water would be better. WD40 seemed to work but just so-so. It leaves a little bit of film. Very mild soap with plenty of water works okay if it is not terribly dirty. I tried dishwashing liquid and did not get good results. I used liquid handsoap and got slightly better results. Stay away from harsh soaps. So, what if your Lexan windshield or faceshield is in very bad needs of cleaning? The Lexan I had was a mess. What worked best for me: LOTS, AND I REALLY MEAN LOTS OF WATER
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MILD TOOTHPASTEThe toothpaste I tried was Aim. (Aim is not as abrasive as some other toothpastes). It not only cleaned the Lexan, but it also removed some small surface scratches. USE LOTS OF WATER.
DO NOT RUB IN CIRCLES. Very lightly apply with lots of water. Rub, ever so lightly, side to side.
DO NOT USE PAPER OR PAPER TOWELS.
Clean, soft cloth, such as an old t-shirt should do the job.
Be sure to rinse with LOTS of water.
I saw someone said use Pledge on it, to wax it, but I would avoid that. I tried it and the results were bad. Always check manufacturers instructions. If you are not sure how a cleaning material is going to work, test a small area, not in the line of sight, first. The toothpaste idea should be tried for Lexan that you think is beyond hope, before you consider buying a replacement. Every few years, you may still need to replace a windwhield or goggles, but if this can add another year of life to an old Lexan windshield, it is worth trying! Note: I did not try more abrasive products such as rubbing compund to see if it would get rid of some larger scratches. If I did, I would probably want to flush it with lots of water then use the toothpaste as a final cleaner). Some windshields are not made of either glass or Lexan. I think I have an old Lucite windshield in the garage. Hmm. I wonder if toothpaste works on Lucite? Will see! | 
