Matt
The problem is with the mould, the silicon had air bubbles is it, when you apply the pressure during casting it forces the resin into the air bubble holes..If you used Pinkysil it tends to set faster, than some other silicons so that air bubbles are often a problem.You can de gas the silicon under vacuum or try pouring it from about 8 or 900 mm above mould.Also mix the silicon slowly.