Options for Underpainting Paints

Ada Leenheer asks: Hi James I like your use of casein as an underpaint, but here in the Netherlands a casein paint in tubes is unavailable. Mixing pigments with powder or liquid casein base is not ideal. It needs a lot of rubbing. Therefore I use self mixed egg tempera = egg yolk, water and pigments. I use this as underpainting for oil. But when in use for putting gouache on top it tends to dilute again when working with a lot of water. Do you have a tip? Could use acrylic medium but thats maybe to smooth to keep the gouache stuck.

Ada, Don't worry if you can't find casein. You don't need it. One replacement solution is to use something like Holbein Acryla Gouache for your underpainting. It is water-based, dries matte, and will not reactivate when rewet. But if you're priming large surfaces it could get expensive. 

You could use artist's acrylic with a matte formulation. Or you could just mix a little acrylic medium with any other water-based paint such as watercolor or gouache, and that will keep your underpainting from reactivating. Sometimes I use a little dab of transparent acrylic with matte medium if I want to accentuate the texture of the unprimed surface.

Or if you want to save a lot of money, or you could even use latex house paint or latex primer, which is relatively cheap. 

A home store will even mix a supply of matte latex house paint to the exact color you want. You can get a liter of custom mix for the price of one small tube of artist-grade acrylic-gouache.

Look for paint that is relatively flexible, and that dries totally matte and not glossy. You don't want a glossy base later because, as you've suggested, your gouache will bead up. As always, if you want to experiment with non-traditional materials, try them out first on a separate scrap and you can head off problems.