I've been refurbishing a kayak paddle (blades) by sanding back and applying coats of XCR laminating resin. All has been working well until I tried to apply multiple coats without the full 24 cure in-between.

Amine blush. It's a reaction between the amines in the hardener and moisture/carbon dioxide in the air. There's various types of amines, and some of them are much more prone to blushing than others, which is why some epoxy products may be deemed 'non-blushing' - they use an amine product that is much less sensitive (or in some cases, don't use any amines at all).