Can Winterstone be used for outdoor sculptures?

Yes.  A Winterstone sculpture left outdoors when properly sealed and maintained can be expected to have a lifespan of many decades. 

Do I need to do anything different for an outdoor sculpture in Winterstone?

Extra care and caution should be taken throughout the whole process.  All aspects are critical - using minimal mixing water, proper and adequate curing, proper embedment of reinforcing mesh and particularly final surface finishing and sealing.

Any specific things to do differently?

1. Reinforcing the bottom of the sculpture for anchoring purposes and properly sealing the bottom to prevent migration of water/dampness upward from the sculpture's foundation.  
2. Extra reinforcement at points of stress in the design.
3. Placing mesh reinforcement as close to the surface as possible without affecting the sanding or surface refinements.

Should an outdoor sculpture be solid Winterstone?

Making the sculpture solid adds weight and not necessarily strength.  The less mass there is the less thermal stress the sculpture is subject to.

Can I use Winterstone for fountains?

Yes.  Winterstone sculptures can be used for fountains or indeed, for sculptures that will live underwater!.