The only guaranteed way to avoid the bubbles is to "de-gas" your casting compound before you use it. We do this at work in a small vacuum chamber (essentially a thick goldfish bowl with a vacuum line attached) takes about 10 seconds and you'd be surprised how much gas comes out.

For home use you could get a hand operated vacuum pump from a school lab supplies company and simply connect it to a small vessel (like a champagne bottle that can stand a bit of pressure - you don't want to implode a glass container, it isn't nice). Put your mixture in the bottle leaving at least 80% air space (you can generate an awful lot of resinous froth and you don't want that in your vacuum pump) and evacuate it , then pour into your moulds and hey-presto, bubble free castings.

Cheers, Sim