When an adhesive’s label says it bonds plastics it normally doesn’t mean Polyethylene.

3M is offering a an adhesive called 3M Scotch-Grip TM 4693 H Plastic Adhesive, and the company states it works for various plastic materials, Polyethylene included.

We tested this product and found that indeed it bonds with Polyethylene, and apparently a little better than other adhesives we know.
However, in our opinion the bonding is not sufficiently strong to allow use in structural repairs or for attaching accessories to a Polyethylene kayak, except maybe a fish finder, a decal etc., which are not required to support weight or resist even weak pulling forces at any time.

This adhesive could be used to seal rivet and bolt holes above waterline (E.G. with carry handles, rod holders, eyelets etc.). We would not recommend using it for sealing holes below waterline because we’re against drilling holes in kayak hulls below waterline in any case…

We would not recommend to use it for fixing cracks anywhere in the hull, and as far as filling gashes (such as those created by oysters) we don’t see the benefit of it, and we don’t see how the adhesive would stay in its place if it had to resist abrasive forces.

All in all, we found that common, general purpose adhesives such as the popular ‘Goop’ perform reasonably well too in such light duty tasks.

In sum, this adhesive cannot replace ‘hard’ methods such welding, riveting and bolting, but it can be useful in other ways.

When it comes to repairing scratches in polyethylene kayaks the methods are similar and depend on how deep the scratch is.

For superficial scratches we don’t recommend any treatment, but if you insist on doing something you can just flame the scratch using a hand-held, propane blow torch. You should apply the flame over the scratch slowly and cautiously until it disappears or diminishes considerably, while being careful not to overheat the area so as not to cause a local deformation. In any case, flaming alters the color of the polyethylene to a darker hue.

For deep scratches or ‘grooves’ it’s better to heat the end of a metal spoon and apply the hot tip gently and cautiously along the scratch, thus ‘welding’ the surface. Here too, you need to be careful not to overheat the area you’re working on since this would cause the polyethylene to deform. You’d need to protect your hand that’s holding the spoon with a thick glove since metal conducts heat and you might get your fingers burnt.

Keep the work area free of any flammable materials and make sure you’re not accidentally directing the flame at yourself or at other people. Don’t allow children or pets nearby.

If you’re not experienced in working with a propane blow torch you may want to reconsider such a project because it can be dangerous.

As for cracks in a polyethylene kayak, those are rare, and they must be properly fixed. Just flaming or welding won’t be enough to fix a crack, and you’d need to patch it – preferably with an internal patch that you’ll weld over the entire area. This is necessary since even if welded the hull in the cracked area will be weaker than in other places, and it could reopen while you’re paddling your kayak or fishing from it – with dire consequences. If the crack appears above waterline you can reinforce the patch with rivets, but we recommend not to use rivets when making repairs below waterline because we think that drilling holes in the hull below waterline is simple too risky in the long run.