Hi Osgood
I have always used Ronseal polyurethane varnish ever since I had a disaster with Humbrol many years ago. The Clear Satin was my preferred version but it is getting more and more difficult to get hold off these days but I did get my hands on a tin of gloss. I let it down with Precision Matting agent to take the gloss off but transfers and lining can still look a bit bright so I add a 'dollop' (about a brush full) of Humbrol Satin Black to the mix to tone everything down and act like a light dusting of soot and ash over the loco. It gives me the finish that I like and others have also commented that it looks natural.
Hope this helps
Sandy