RainX. Tried that and some of the other types that are supposed to do the same job. Now I have about 4 of them and as far as I am concerned none were worth the money at about a fiver a toss! They might deflect the rain for a while but I found that if you have to use the wipers it starts to smear and the vision is affected. Its a real bugger to remove it too and I ended up carefully using cellulose paint thinners cos it was the only thing that worked.
I have now gone back to the old way of using coarse paper (newspaper) and warm water with a splash of white vinegar in it. Works every time and gets rid of all the traffic film and greasy spots. A quick polish with a bit of clean paper and thats the job done.

After you have cleaned the glass
do not forget to clean the blades too or they will just put back on the muck you have spent time cleaning off!
Sometimes the old ways just cannot be beaten...................there again I am an old bugger anyway
