Indeed, a single piece of gaffer has been stopping the crack in rear window on my MX5 from leaking for a couple of years now. When I left the car standing at the airport for 3 weeks last year I taped along the tops of both doors so that the holes in the roof didn't leak for 3 weeks solid with no chance for the inside of the car to dry out...

I really must get a new roof !
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Remind me to change my signature to something more interesting someday