Hi, The approach I use for my 15s, especially the white ones, is to combine the shading and gloss varnishing into 1 step.
Polyurethane gloss varnish, plus white spirit to thin it and a small amount of matt black enamel - I use Humbrol. You need to decide what amount of black works for you - start with a little and add more if the first figure or two doesn't seem shaded enough.
I've used it on Biblicals, crusade muslims, Austrian Napoleonics, Mahdists and others, and I'm happy with how it turned out. One day I might get off my lazy a**e and put some photos up.