Plastic.. its sure.. thers a problem, when working with foam.. there must be one side that absorbs any material.. so,i just apply white glue over the surface before PS grey.. for water models can use Marine glue, is also water based.. or can also use acrylic wall paints or acrylic wall putty, are also water proof.. this gives a fine coating that prevents any absorption and also gives a smooth surface.. then we can apply a little polyester putty if needed.. its easy.. and inexpensive..
