resin hardens

A Quart At a time??!! Holy crap that's way too much at one time! I live in Florida and my batches are limited to about 6 or 7 oz . I work outside in the sun, as I don't have a garage. The resin I use asks for 8 drops MEKP per oz of resin. I usually only use 6 drops and still have a small amount  that hardens before I can use it all.

Another thing to consider is what brand resin you are using. When I first started fiberglassing I used West Marine resin (which sucks by the way) and only got about 10 minutes work time out of it. But Us Composites B-440 resin will get you about 30 -45 min work time. Again it depends on how much MEKP is used, outside temps etc.

And how do you think the UV light makes the resin go hard? Its because UV light can be used as a heating source.  The resulting reaction is exothermic

When you mix you MEKP hardener with a polyester resin, it lowers the activation energy of the styrene monomer in the resin.  This then means that the resin is able to set at lower temperatures.  If you expose a resin mix to an isolated heat source it will eventually cure (will be very brittle thou becuase you are forcing the monomers to bond).  When your working in the sunlight you are working in the heat.  Hotter conditons means that the resin sets faster as there is more free engery at elevateed temperatures.  Colder wokring conditions require more hardener to further lower the activation engery.

I appreciate your knowledge of physics, but...

a black light is a UV light source. Have you ever felt a black light get hot? Probably not. I understand the whole MEKP (methyl ethyl keytone peroxide) concept. I will however contradict what you said. Experiment. Mix a batch of resin and spread it over two different projects. Place BOTH in the dark and use a black light over one. UV from a black light does not travel far, so the two can be in the same room (garage). They both will cure, but I bet my left one the UV assisted project cures first.