: The epoxy application was kicked-off when I checked this morning. I
: measure the resin and the hardener separately (accurately) and
: mix for 3 full minutes.
: I have already experienced poorly mixed epoxy in the past and had
: to scrub it off, so I am extremely careful when mixing.

: Without knowing the whys, I'll not use any hardener that has
: discolored in the future.

I used some WEST 206 which had sat on the shelf for at least 15 years and had yellowed somewhat. It set as well as the new stuff. When I rang the supplier who makes WEST here in NZ (under license), I got to talk to the company 'chemist' (as he was introduced to me!) and he said officially it has a shelf-life of 3 years or so, but unofficially it should last a very long time, if kept at a reasonably constant temperature and low humidity.

You should at least give the product the benefit of the doubt. It's such a plausible explanation, that you just might have gone a bit light on the hardener.