Some of these slow hardeners are really S_L_O_W and are designed for boat people who do summer repairs in 80 - 100 degree temps. Chances are that if you put a heat lamp on it it will cure up pretty good. I would do a test on a small scrap and put it somewhere warm like next to your furnace or out in the bright sun with some plastic over it and see how it goes.

I once used RAKA slow hardner and went through the same thing. I called and they told me what I'm telling you.