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Fibreglass doesn`t usually work on plastic butts - either because the butt can flex slightly (which the fibreglass resin can`t), or because of the differences in the respective coefficients of linear expansion of the two substances. At any rate, I`ve tried it on 3 different kinds of butts, and it always cracks away from the plastic. Try Duck Tape, or waterproof glazing tape applied to the inside of the butt, or use a heavy grade wheelie bin liner.