A 17ml bottle of Vallejo Model Color 520 Matte Varnish

A quick-drying water-based matt acrylic varnish with great hardness and permanence. You should let the underlying paint dry thoroughly (at least 24 hours, preferably longer) before varnishing. Best applied in several thin coats than one thick one.

Note that Vallejo re-branded their varnishes a little while ago to make them generic to all their ranges (Model Air, Model Color and Game Color).  They finally realised that it was the same stuff in the bottles, but with a variety of labels !  All their varnishes are suitable for application by brush or airbrush, and shouldn't need thinning for airbrush use unless you are using a nozzle smaller than 0.3mm or a higher than usual air pressure.