I use the Mer Rain away stuff to great effect
the trick to applying it and having it work properly is two things, cream cleaner and a microfiber cloth!
Use cream windscrean cleaner to deep clean the pores in the glass, let it dry to a powder and buff off with a blue microfiber cloth from halfords. Then apply the rain off to the freshly cleaned glass, let it set and then buff it off with the microfiber cloth, then reapply it and leave the second cote on for 24 hours or over night if you dont have the time and them buff off, again with the microfiber cloth.
This prosses means that the stuff bonds and seals the screen, the first coat bonds into the pores of the clean glass and the second coat seals. the microfiber cloth means that it will remove the smears with no effort needed.
if you do this then sh1t like bugs and bird gunk wipe off with ease and no smearing with a little blast of the washers.
I know coz i used to sell the stuff for MER!!!!