Rubberised raincoats often suffer from defects later in life: cracks, holes, or flaking of the coating, results from wear or ozone.
But if two rubber coats are worn at the same time, even if both have such defects, the pair will be very nearly 100% waterproof even in a heavy, sustained downpour - because the defects in the two coats are almost certainly going to be misaligned.
Therefore, don't toss out or try to repair a slightly damaged mac - just slip on a pair!
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