I had to do a kayak repair and dug out a quart size plastic bottle half full or Raka resin that was as hard as a rock. I figured that perhaps I put it in an old bottle that had hardner in it? It was like a rock in a bottle.

I put it in the microwave for a minute and I could see liquid forming. Another two minutes and lots of liquid. I put it out in the sun (85 degree day) and it's all liquid and works perfectly. The hardners never seem to crystalize like the resin does.

Working outside in the sun, my boat surface must have been 120 degrees. Using Raka fast hardner I made some putty and fixed a cracked strip and immediately glassed over it. The epoxy kicked off and in an hour I was putting on a fill coat. I like the fast hardner for repairs or bonding things although it's not blush free.