Paint it brown (courtesy of Anita's Acrylics Earth Brown) and when dry cover base in PVA/wood-glue and add ubiquitous "gravel and sand". Then let set (see below):
The base is now "dusty and sandy" and almost impossible to paint on. A watered down wash of Anita's Acrylic Earth Brown, with a little bit of black added, is then soaked into the base to seal the "grit". A touch of left-over PVA can find its way into the mixture.
Note: The metals are heavy so when the cardboard base was sodden, care had to be taken with it when moving it about. Super-glue was used to stick the horses to the card-stock.
Getting there! Next stop: A bit of flocking.