Deciphering the instruction- question

[cfuruti]

It depends on how thick is your paper/card and which paint you use. For instance, although it's water-based, acrylic paint dries very quickly and covers well enough to avoid warping of thin cardboard - but I have only used it for edge-coloring and retouching, not to cover large surfaces; prior to painting, several members of this list spray some kind of varnish or lacquer, which covers (sorry) this case.
I've never used an airbrush, but IMHO a flat paintbrush would suffice.