On your site you list an acrylic modifier for gypsum cement/plaster. Would this product waterproof the casting mixture or do you know of other products to mix into the batch for waterproofing the casting, which will be mounted outdoors.
The Acrylic Modifier for Plaster is meant to increase product hardness and chip resistance, not to waterproof. Even if you were to dip-coat the cast with any of our acrylic resins, you would get water resistance, whereby water/moisture won't readily pass through it, but standing water (puddles of water, use in a pool of water, etc.) would eventually work its way into the cast.

Potentially, you could achieve this with a resin system like a Polyurethane, urethane or epoxy for outdoor use, but we would think for practicality sake, one of the newer resin casting materials would be better.