I used RainX once and didnt really care for it. It is, I believe, a
silicone compound that
adsorbs upon your windshield glass and makes it shed water quickly.

As mine deteriorated, I had some issues with patchy spots, and decided if I
ever got it
clean I would never use the stuff again.

You spots may be from oil from the engine or highway, dirty wipers, paint or
chemicals
which have drifted onto your car or maybe even more possibilities.

You may have to clean the glass with a good glass cleaner and replace your
wiper blades
to get relief. (I have some windows that are so badly filmed with
"something" I have never
been able to get them clean.)