AMMO by MIG Accessories - Putty For Modelling
Item: Arming Putty Classic

From the Manufacturer:

If necessary, it can be diluted with Extra Thin Cement A.MIG-2025 or with Odourless Enamel thinner A.MIG-2019 and applied with a brush. This formula will not shrink and dries in 24 hours.
It can be sanded to obtain a smooth and very resistant surface ready for engraving and painting.
Clean your brushes and tools using the Odourless Enamel Thinner A.MIG-2019.