I need to airbrush a quick drying matt varnish over an inkjet decal. I normally use Humbrol aerosol varnish but it has been discontinued. I have been recommended Humbrol MattCote in the little square bottles. Can I airbrush this as I don't want to drag the pigment on the decal? If so what do I thin it with? It is going over tamiya acrylic.

I use the humbrol matt in the tin. As it is an enamel I thin it with enamel thinners. If it is an enamel one in the bottle then it should go over the tamiya.

I thin it with humbrols own enamel thinner. Works good for me and should be not problem over tamiya paints.
You will get lots of suggestions for different thinners ... but you need to experiment with them first as they all react differently. For suritys sake .. go for the humbrol thinner.

I've also used Humbrol matt in the tin. It's the flattest matt I've ever used. I thin it with Humbrol thinners and it airbrushes really well.

I always test it first though. I once had it turn white and powdery, so these days I spray a litle onto something black and wait until it's dried before applying it to my model.