It can probably be thinned with water sufficiently to allow it be used in the airbrush. You'd need to do some trials to achieve the desired thinning ratio, but as a guide, I normally thin my matt acrylic varnish at roughly 50/50. to 60% water to varnish ratio. If it seems OK, try it out on a "hack" first, before committing to a model.
Alternatively, you could try the type I use, which works very well. This is the
Vallejo Matt Acrylic Varnish, and it sprays very nicely, giving an even coat over enamels.
It's readily available in the UK ( I got my first, small bottle from Model Hobbies, which has lasted a couple of years so far ) and comes in 60ml and 500ml bottles, the latter being more cost effective.
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