Hi PJK
Out of curiosity whose acrylic paints and pigments are you using for the weathering?
Another vote here for the Alclad varnish range. My local model shop started stocking it so I gave it a try, it's consistency is such that it goes straight in the airbrush without any thinning. I tend to use the semi-matt/satin varnish rather then the full matt.Now I use Alclad matt varnish.