How to Repair Plastic Gutter Seams
Wash the seam area of the gutter.
Clean the repair area to remove any grime or debris.
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.
Apply a bead of caulk along one of the cracks.
To apply the bead of caulk, move the caulk gun at a measured pace along the crack and using continuous light pressure on the gun trigger.
Smooth the caulk.
You can use a craft stick, plastic spoon, or wet finger to create a smooth surface.
Caulk the remaining cracks following the same procedure.
Caulk on all cracks between gutter pieces.
Clean the caulk tube.
Wash away any caulk remaining on the gun, your hands, or other unwanted places before it dries.