For what it's worth, make sure the surface is really clean before you apply the Rain-X. I like Stoner Invisible glass. It has no ammonia in it and no detergent to leave any residue behind.
Then apply a coat of rain-x using a paper towel, making sure to apply in overlapping circles. Let it flash off and then apply your second coat. Lightly spray a microfiber towel with the glass cleaner and then use it to wipe off the haze. Then buff it with a clean microfiber.
Now, take a used dryer sheet and lightly wipe the inside of the windshield.
Since the windshield is plastic and not glass, there is going to be a static charge build up. That is going to help the water stick to the windshield. The dryer sheet sill help reduce the static charge and allow the water to flow freer.
Instead of rain-x, you can also use spray wax. The kind where you spray it on while the car is still wet. The water helps disperse the wax agent and when you wipe it off you buff it in. And each time you wash and use it, it builds up protection. Just carry a dryer sheet with you to use as needed.