Try the white "Crystaline Basecoat" by Too Much Fun available from Thenailwarehouse.net.
If you are using quality opaque airbrush paints you will not need to airbrush white as a first coat unless you are spraying over an acrylic pink and white. Moral to that story is don't either encourage client to have clear, natural or just pink acrylic or airbrush a very dark design for full coverage in one easy step.
Pat