Standard procedure to remove rain repellant stains from windscreens was to use Coke Cola. However, I never saw it on the approved chemical list either
Flew the 737 for years in the Pacific where blinding rain sometimes meant a diversion of 3-500 miles to possibly similar weather. Rainex was superb and never had problems with windscreen serviceability. The installed rain repellant was also good if the nozzles didn't clog which they often did. If we gave a squirt of repellant we logged it in the maintenance sheet so engineers could clean out each nozzle.
It was well known since the first 737 rolled out that the windscreen wipers were not only annoyingly noisy but also could not cope with anything apart from light rain. The Coca Cola remedy for cleaning away the residue of the installed repellant was excellent. Used that for years with no serviceability problems.
Of course you can always depressurise and open the side windows for a better view and old aircraft like the DC3 and DC4 had storm windows which you could open and get an excellent view through the rain or iced up front window.