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Will it light up with a spark after it gets wet?

dunno. Maybe I'll try out of curiosity.

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I wonder if it will seal deet and other rather active substances?

dunno again. My guess is it would. Easy to test. It is very chemically resistant (acids, bases, ethanol, salts, etc) except for things like toluene, chorinated solvents, benzene.

It does melt at 60^C!

chemical resistance of parafilm