Seal and Release.
If the model is porous like stone, wood, plaster, or concrete, you will need to seal it first – make it waterproof. You can do this with paint, shellac, paste wax, etc.
Then apply Pol-Ease® 2300 Release Agent and brush it around thoroughly to get it into all the texture.
Polytek also offers products that are both a sealer and release. Pol-Ease® 2350 Release Agent is a mixture of Petroleum Jelly and mineral spirits. This is brushed onto porous surfaces and left to dry. The mineral spirits will flash off leaving behind a fine film of petroleum jelly that both seals and provides for release of the polyurethane. It will permanently darken most materials.
PolyCoat Sealer & Release Agent is another sealer and release. This one is a moisture-cured product that creates a permanent silicone surface on the porous model. Brush or spray it on the surface and let it dry. It dries clear and virtually disappears.
For non-porous materials you just need a release, like Pol-Ease® 2300 Release Agent.