Use black wash for metals, like rifle barrels, bayonets and knives etc, you could use the black wash all over your figure but then it would be very dark! Instead, I suggest using flesh tone, either from Vallejo or GW on skin parts, face and hands, then depending on cloths, light wash for light colors, I prefer to use Army Painter Soft Tone on lighter colors like white, for red you can use a brown wash like Army Painter Strong tone. For grey you can use a black wash. For green use strong/brown tone, blue looks great with a dark tone. If you'r really lazy you could use just one wash like soft tone or strong tone, I tend to go with soft tone as I don't want my figures to look too dark or dirty.
I really suggest you invest in the small Quickshade bottles from Army Painter, buy soft, strong and dark tone, and use the dark tone on weapons, it's basically a black wash. There's nothing wrong with Vallejo but from my experience the quickshades are really better. As for varnish, don't use it, if you must before the painting, as the varnish will mess upp the effects from my experience.
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