Q: What is Hydrostone, and how should I mix it?
A: Hydrostone is a U.S. Gypsum product which is considerably harder than regular plaster of Paris, and lacks the period of plasticity in its setting curve which allows plaster to be worked directly. Any surface cleanup should be done immediately on demolding, as it will continue to harden for several hours thereafter. You can add considerable amounts of filler to hydrostone. Don’t mix this with your hand, like you can with regular plaster of Paris. I prefer a “jiffymixer” sized appropriately to the mixing vessel.
There are now alternative materials which can be used in much the same way as hydrostone, but are stronger and more durable outdoors due to the addition of plastic resins. One of these which I have been working with lately is called Fortran MG, and is mixed from gypsum cement, a polymer emulsion, dry melamine resin, and hardener. This may be filled with metal powders and patinated, and it accepts paint well. It may be laid up like a polyester/fiberglass composite, but is not nearly as toxic.
All plaster products should be mixed thoroughly, for at least two minutes. This helps avoid water separation, which will destroy the surface with dendritic trails, and improves the working properties. You don’t have to sift plaster with a screen. If it contains hard lumps, it has become hydrated and should be discarded. It is enough to add it slowly to the water with a scoop, shaking it off the scoop into the bucket. As when using any powdered material, a dust mask should be worn when mixing. Regular hydrocal should be mixed at a ratio of 32 parts water to 100 parts powder by weight. (A bit wetter than regular plaster). “SuperX” hydrostone is mixed 22 parts water to 100 parts powder.
For product literature on their wide range of plasters and gypsum cements, call USG at 1-800-621-9622