Partially cured resin

Not only is it necessary to mix the two parts together well, it is also necessary to shake them or agitate each part separately before adding each part to the mix. I forget which- the resin or the hardener, separates into two parts itself if it hasn't been used in awhile. I had a lot of trouble with resin castings until someone pointed this out to me. Since agitating both parts before mixing, I have not had the problem.

I forgot to mention one other possibility.  Some folks try to make an open, uncovered mold with just a depression that the resin is poured into , with that surface left open to the air.  That doesn't work.  The resin cures by heat from the reaction of the two chemicals.  When a part of the resin-hardener mix is left exposed to the air it does not get hot enough to completely cure.  So, if you make such a mold, place a piece of particle board scrap or something over the top when you are done pouring, so that resin surface is not exposed to air.