The only thing that has, and ever has, worked is Windowlene (or the more expensive autoglym fast glass – but why) and elbow grease. The elbow grease bit is important.
I use old ripped up bedsheets, the cold smooth type (not the fluffy cotton type), spray windowlene on the screen (I do it in blocks of 1ft from bottom to top, moving across the windscreen) and scrub f****ng hard with the cloth. You need to scrub until no trace is left on the screen. You will notice the cloth turns quite black – that’s all the sh**.
It will be factory spotless. You then shouldn’t need to clean the inside of your windscreen again if you do it properly, indefinitely, until some moron touches it. Never, ever, ever, ever, ever, wipe condensation off of it. Sit in your car and wait out the 5-10 minutes for the screen to clear when it’s cold outside.
ELBOW GREASE.
ELBOW GREASE.