Dealing with Frosting
One of the problems with Citadel Satin Varnish is that it 'frosts', i.e. deposits a white crystaline material on the mini - often in pretty patterns - see below.
The cause is uncertain because it doesn't happen every time, even with the same can. Folk lore has it that air humidity plays a part but it is difficult to see how this can apply to Southern England in September (Temperature about 15C).
I posted the pic below a couple of days ago and received useful advice from Da Masta Cheef on removing it by the application of a little olive oil.
This got me thinking (I am a research scientist, after all). I tried a little test. I applied a little water to the gun shield and sure enough the frosting disappeared, only to reappear when the water dried. The varnish must have a crystaline irregular structure that appears white on the macro scale. The water fills the irregular gaps and presents a flat surface to light reflected off the model - so no white appearance.
Anything that achives the same effect will remove the frosting. In the search for a permanent cure, I tried spraying the model with Humbrol varnish, designed, I assume, for enamal paints. It gives a strong, thick, permanent finish. I would advise only doing this on well cured acrylic paint (but acrylic varnishes would probably work just as well).
As you can see from the pic above, it works tolerably well.