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.

Iv had the same issues number of times,  keep getting told it's mixing ratios,  which I can't see how it can be as I use digital scales.

The only reason for it that I can see is blushing. I'm not sure if the XCR is supposed to be non-blushing, I imagine it would be. But in most cases the only reason an epoxy won't bond to itself is if it develops amine blush on the surface and the salts become a contaminant that prevents a good bond. 

What's blushing?

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). 

Just took a look at the TDS, and it appears the XCR is in fact very sensitive to amine blush. 

Just at home, I don't have access to workshop conditions.

I would also tighten up on your weighing and mixing as slight errors in these can all contribute to the problems you are having.  When the mix ratio or mixing itself is out, the cured hardener components can come to the surface as with blushing causing the same problems.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

I am using digital scales and mixing up 12g at a time (9g resin, 3g hardener).

Ah, see small amounts are very difficult to measure accurately enough. With epoxy you should always be within 1% accuracy. For 12g, that means you can only be 0.12g out. Are you confident you were within that margin?