How to Repair Plastic Gutter Seams
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Prepare a tube of exterior-grade polyurethane caulk.
Cut the tip off the caulk tube nozzle at a point where it produces a bead large enough to fill the crack. Start small — you can always make the hole bigger. A 1/4-inch bead is large enough for most cracks. After you cut the tip, you have to puncture the seal in the end of the caulk tube using a stiff wire or long nail.