Rain X is quite a mild compound as far as rain repellents go.
much more potent products are available such as the previously mentioned H2go and Gtechniq G5 which i use..
Halfords duxback treatment that was asked about earlier is a semi permanent compound that should last around 6 months before effectiveness drops off
Problems arising from using these products nearly always comes down to the same 3 issues ;
applying too much product – use the absolute bare minimum
pre clean not thorough enough
wiper blades worn
anyone wanting to remove a failed treatment should try scrubbing the glass with fine wire wool dipped in white spirits – guaranteed to strip back to naked glass.
HTH.