Game Decor has launched a Kickstarter campaign in order to fund a new line of paints specifically formulated for terrain. This is their PaintWorks line.
From the campaign:
Game Decor’s PaintWorks paint kits are designed and formulated for use with plaster, resin, and/or foam terrain for use on game tables. The paint does not require the terrain to be primed or sealed before the base coat is applied. The high density and high quality pigment means that in most situations you only need to apply the base coat once to fully cover the underlying model in a single coat. A single basic (3 color) PaintWorks TerrainPaint set will cover approximately 8 square feet (actual covering area will vary by how porous the surface of the model is).