PolyFibres from Polytek are added to Urethane resin, rubber or silicones (you lose transparency if you add them to the Platsil gel range of silicone) to create a brushable paste.
This lets you create brushed up molds and casts.
 
You can create lightweight casts in brushed up thin layers. The more fibres you add, the thicker the material becomes.
 
A 5% mix by total weight of mixed liquids will give a highly thixotropic mix but you can add less or more depending on you requirement