: I thought it was the other way around, warm and wet is loosest?
: Kayak I skinned in January when it was freezing gets loose now
: on a warm wet day, the one I skinned in spring stays tight. I
: use a regular spray bottle to wet the few feet I'm sewing. Wet
: the whole thing before stretching over the stem and stern.

Thomas, you may be right

Warning, science content:

A very common recommendation is to sew nylon when it is cold and wet. Wetting nylon makes sense. One of the properties of nylon is that it absorbs water and relaxes as it does. Water molecules take up space and displace the nylon molecules, causing swelling. As well, the increase in distance between nylon molecules reduces the forces holding the nylon together, allowing it to stretch more easily. The process is reversible so that nylon tightens up as it dries.

The recommendation to sew when it is cold is however difficult to understand. Nylon has a positive coefficient of thermal expansion. That is, like most materials, it relaxes when warm and contracts when cool. A nylon kayak skin should therefore be most relaxed when it is warm and wet.

The recommendation to sew nylon when cold and wet appears at odds with the known properties of nylon. The only explanation that comes to mind for this widely made recommendation is that in the places where kayaks are being built cold days are more likely to have higher relatively humidity. What perhaps is being observed is not that nylon is more relaxed when cold, but more relaxed when the relative humidity is higher.

If this is correct then nylon skins for kayaks should be kept warm and wet for sewing.

Jon Mortimer