Just use micro glitter...like glitter dust. Also, don't put too much in or you will have settling. TGB
That was going to be my next suggestion. I have a couple glitters that cast fine no matter what I do, but my favorite effect was the mirror men I made and the glitter is so big and heavy. But so so pretty. I love those crystal clear with glitter.
You can use larger glitter...just add it when the plastic is gelling and it should suspend better, but, your chances of not getting it pressurized in time are increased as would be air bubbles. TGB
