I would want to get the glass completely free of anything before I applied any glass coating product on it...
Rain-X has something like denatured alcohol in it already, so it should be pretty good on already cleaned glass..
Not sure why one would apply another glass coating over a previous coating - unless the coatings are the same and you are renewing the coating..
If it is possible to get the glass really clean, it will be really smooth to the touch, and that alone will allow it to shed water.
I think once the glass pores get full of gunk, it does not repel water as easily.
The newer glass coatings mentioned above would be a great place to start if you are looking to really improve from what Rain-X is doing..
I have always had great success first cleaning and then using a glass polish on all the glass to further clean and get it really smooth..
If you can do this with a Random Orbital, a smaller pad - 3-4", and a good polish, you are going to have some beautifully clear glass that is going to repel a lot of dust, smaller flying things, and water really well !
And if you lived where I just lived, you really want the water repellency..
Dan F