Rain-X was developed for plastic aircraft windshields because they do not have wiper blades. i have used Rain-X since it came out and never had a shield or my plastic eyeglasses fog.now it may fog if you don't buff it completely.
Big red one has the best answer. Novus 1 is very good. If you want to use a repellent Harley makes some presoaked towellets which work well and designed not to hurt the LEXAN your windshield is made from. No RAIN-X.
I use a product called Nanowax made by Eagle One. Its a spray wax with no residue. I use a microfibre towel and clean the whole bike with it. Only time I ever put water on the bike is when it gets muddy and thats not to often. Takes the bugs right off the windshield. If it gets on the seat it wipes right off. I do the chrome at the same time. I buy 4 spray bottles at a time. I spray it on the shield every time right before I ride to clean it. I have been doing it to the stock shield since I bought it. The dealer had given me a sample pack of the chemicals HD recommends and there were 4 different products and they are not cheap. I think I give $6.99 a bottle for the Nanowax and I only use it for everything. IMO I wouldn'tbuy anything else.
I ride a Road King, so I just remove my windshield and throw it in the dishwasher once a week, used with a little jet dry, I don't have any water spots and stays really clean!