Gear Aid Seam Grip Sealer & Adhesive. Make sure its the one for PU coated fabric and NOT the one for sil fabric as it won't work.
Once opened you can put the half used tube in the freeze so it won't go off and it should last for years. I have a tube that's at least 6 years old and still usable in the freezer. I use it to fix small holes in my packraft aswell.
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