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Does everyone use teh GW varnishes and if so whats the main advantages of either the satin ones over the gloss and vice versa?
Here is a general rule of thumb - the glossier the varnish the more protection it will offer.
Now on to recommendations:
Gloss varnish will leave your model looking very shiny. Usually this is not a good thing. What I suggest is to use gloss or satin (less shiny) in conjunction with a flat matte varnish such as testor's dullcote. The flat varnish will kill any shine caused by the gloss or to some degree the satin varnish.
It is in my opinion that models look much better without any glossy reflections. If you can't find testor's dullcote (I think it is one of the best) - there are other brands of flat matte varnish out there.
Hope this helps.
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Just to second what slorak just said. Testors dullcote is my choice of varnish.
Glosscote & Dullcote are my favorites as well. I've played with both the spray and brushed varieties and find that the sprays tend to work better for me. When I use the brush on varnishes, I end up smearing a little bit of paint around. This could simply be from not keeping the brush clean enough. Be careful if you go that route, however.