Patsy,

I am of the opinion and experience that if you exceed 6-12 hours between coats of epoxy, you need to sand the surface. This opens chemical bonding sites on the previous epoxy, so once you sand you should epoxy right away (or at least before 6 hours).

If you do not sand (I didn't around my kayak cockpit opening when I glassed the rim) you may find you have a thin feathered layer of epoxy which looks good until something knocks it or you sand it. Then you will find that the two coats did not adhere to one another. This will happen with both non-blushing epoxy and normal epoxy.