I want to put a coat of flat varnish on my latest model to help hide the decal edges and remove the glossy effect of the paint I used. How is flat varnish best applied to an armour model, and if you use an airbrush, will the varnish hurt the airbrush if its internal mix? Also, how do you thin the varnish for spraying? I've been told to use white spirit, but aren't too sure about it.