Glass, Perspex and Lexan are prone to build up of salt and other residues that are hard to remove with spray glass cleaners. Sea View is a heavy duty cleaner that contains ultrafine abrasives specially designed for use on Glass, Perspex Lexan and Chrome.
Just put some product on your cloth start in the centre of the window and work your way out. You can feel the cloth dragging on the salt etc. keep rubbing until the cloth glides freely. Then remove with a clean cloth.