I've been painting a series of master copies of Baroque portraits, and I wanted to emulate the double-toned ground seen in Van Dyck's Brussels Magistrate series of head studies, which show a warm gray thinly applied over a dry umber tone.
I started with Raymar's Claessens C13DP Linen Panel and toned it with a layer of burnt umber. It works best to use a fast-drying, binding medium so I used an alkyd medium to increase the flow as I spread the paint. To get the pigment down into the grain of the linen I scrubbed the paint on with short strokes of a large filbert brush.
When the burnt umber ground was completely dry, I mixed a little raw umber and bone black oil paint into Rublev Lead Oil Ground to make a warm gray. My tone ended up a little too light and too cool, so when I do it again I'll omit the black and add more raw umber to warm and darken the tone.
I also left a little margin of the red ground showing around the edges, which makes a nice vignette effect surrounding the head studies.
The benefit of using a lead oil ground instead of lead oil paint is the ground mixture also includes chalk which gives the surface a slight "tooth" and absorbency, a surface that is less slick than if it were primed just by applying straight oil paint out of the tube.