I have always used a product called 'Envirotech' which is a 2 part epoxy that you mix when you're ready to pour. There are more products on the market now than when I started using it in the early '80's for my miniatures. Here's a web site that has a couple of choices, I buy mine at WalMart now, used to have to go to a hobby or hardware store.

Here is a link that might be useful: Resin