Hi, I have hot coated a kiteboard to test before doing it on a bigger job. 

The standard mixing ratios for all our epoxy resins are based on parts by weight, so you will not need to take into account density and temperature in the weighing process.  A minor change in the ratio could easily be the cause as could be humidity.  You mention the humidity was controlled so it should not be that.  The temperature is ok but higher the better. So that would suggest potentially a slight error on the mixing.

A light wet sand and polish should remove any traces.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant