If the old "Rainboe" rain repelant was applied to a dry windscreen it would leave a sticky film on it . This would cause visability problems, particularly in bright sun light. A quick "on the line" way of removing said film was to pour a can of "The real thing" on the screen, then wipe if off. God knows what that stuff does to your insides though!.
It's not an urban myth either 'cos I've done it and it worked like a charm!!!.
Rgds Dr I