I got this “recipe” from a fellow artist, John Hagan, who had been making his own boards since 1975. He wanted to create pieces that were sometimes as large as five feet square and this was the only way he could accomplish that.
PVA...White craft glue.
Whiting... Whiting is a white powder used as a filler in woodworking. It is mined like chalk and ground to a fine white powder, or you could probably use plaster instead. (You can differ from the original recipe and use Porcelain or Kaolin Clay).
White Paint...Water based paint.
Mix 2 parts PVA, 2 parts Whiting, and 1 part White Paint. Paint thicklywith a broad brush on a piece of masonite (keep board flat). Make 4 to 6 coats letting it dry between coats. The glue prevents the penetration of the inks; the whiting gives body; and the paint makes it white. If you experiment you will find the combination that suits you. Sand it smooth and add inks or any medium you choose to work with.