I'm about to try this for the first time myself and from the examples and methods I've been reading and photos I've looked at, I'm going to go with Baking Soda. The way I understand it is to mix it with a little PVA and a tiny bit of white paint, then put it on the base, then give it another coating of baking soda on top, which you can then seal with some thinned out PVA again.
I'll post pics when I finally get to it.