: Polyurethane glues that I'm familiar with expand as they cure. I suggest you
: test on some scrap first to see if you like the results. Several people on
: this board use this glue for edge gluing their strips, so it seems
: reasonable to think about trying it. As to whether it is strong enough, I
: will let the more experienced members answer that question.

The polyurethanes do expand, but of course there are limits. The Gorilla Glue that I use for stripping expands about 2 or 3 volumes in an unconfined space. But the mechanism that causes the expansion is the reaction of the polyol resin with moisture, which creates carbon dioxide, which in turn creates the cells in the foam. Therefore the force causing the expansion is the force of CO2, which is quite low. In a confined space, i.e. a glue line, the size of the cells depends on the thickness of the space, alternate routes for evacuation of the CO2 gas, and clamping pressure. When edge gluing strips it is difficult to create enough clamping force over enough area to prevent the glue from expanding and creating gaps, so a tightly controlled thin line of glue is called for. For scarfing panels together there is no such difficulty, since ordinary clamping methods will overcome the tendency of the adhesive to foam, rendering instead a desirable thin bond line. It is also waterproof. Pay attention to ambient moisture, though, since effectiveness drops off below 80% RH or so. In that case the pieces must be dampened before glue-up.

That said, epoxy is still better. Even in cold temperature. It just takes longer to cure. If it were my boat, all scarfs would be epoxy.