‘Plastic’ kayaks is a term that refers in most cases to rotationally molded Polyethylene kayaks.
Polyethylene (HDPE, MDPE, LDPE and various commercial names) is by far the preferred resin in the kayak industry because of its superior performance when it comes to shock resistance, durability and overall reliability, but it is known to be quasi impossible to bond. This is because Polyethylene’s surface tension is low, which doesn’t allow for significant chemical reactions to occur, including bonding. The upside of this feature is that Polyethylene has superior resistance to strong solvents, acids, radiation etc., and indeed it is used for making fuel tanks and containers for active chemicals.
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.