The primary strength of your craft will come from the material used to cover it. Tensil strength, and shear are a couple of them. The epoxy really just holds the material in a way that they can do their job correctly.

Considering abbrassion resistance might be a concern as to the hardness of the epoxy, but I don't think it has much to do with the stregth of your kayak as long as it is holding the material fibers in their proper positions. If it becomes brittle, then you could have a catostophic failure of the fibers in the material if the material is weak in shear strength, such as is the case for Carbon fiber.

If I'm wrong here, please let me know. This is what I have been informed of.

The closer the mix ratio gets to a 1:1, the more forgiving errors in mix amounts are. I would conjecture that just about everyone who has ever used the stuff has messed up a batch somehow unbeknownst to him or her. Or at the least questioned if they had counted 3 on the up stoke or down stroke....OH Sh@T...I'll have to turn that radio off so I can count to 3.

Hope this helps.

Tom