A mass-production miniatures painting technique that has really taken hold in the last decade or so is auto-shading by using glazes or washes of various formulations. Vallejo and Citadel both produce water-based washes in an increasing range of colours, and when used sensitively they can create excellent, subtle shading with minimal effort.
For wargames figures, less finesse is required. The miniatures are intended to be viewed at arm's length or further, so the shading results can (and probably should) be less subtle. Plus, wargaming figures benefit from being well protected from the vicissitudes of handling by a good, sturdy sealing coat.
Here are a couple of alternatives to achieve that end.
On the left,
Army Painter Quickshade (Dark Tone). Quickshade comes in three strengths of tone, so you can use whichever best suits the underlying colours. This 250ml tin cost me $45, which is not cheap.
On the right,
Cabots Varnish Stain (Indian Tea). This can be tinted in a much wider range of colours. The 250ml tin cost me about $25 — slightly more than half the price of the Quickshade.
However, price is not the only determining factor when one is choosing a product. How do they perform? Both, as far as I can determine, do fundamentally the same job: both are oil-based, fairly low-viscosity, tinted varnishes.
Below are two varieties of Reaper plastic zombies. Each type has been block-painted and then just dipped in the stain — a crude method of application; brushing the stuff on would certainly give you more control.
Of each type, the Quickshaded one is on the left, the Cabots Varnish Stained one is on the right.
The Quickshade gives you a much more subtle shading effect than does the Cabots, which is probably
usually going to be preferable. I haven't yet tried thinning the Cabots to see if that ameliorates the chalk-and-soot effect — if it does, then its much lower price would make it a more cost-effective option. Both products do a decent job; if price is no object, my taste runs more towards the Quickshade outcome.
Both of them harden over about 24-48 hours to a a rugged, glossy finish. That means that unless you like your toys shiny, you'll need to hit them with a decent matte-coat afterwards. Some acrylic varnishes don't work or play well with oil-based underpainting, so that means that the matte-coat should probably be spirit based.
The Next Day...
I gave the zombies a squirt of Vallejo acrylic matte varnish to take the gloss off them, and it went on just fine. It doesn't seem to be bothered by the oil-based varnish underneath, which is good.
Something I've found about acrylic matte varnishes is that they really do work best sprayed. I've never achieved anything better than a satin finish by brushing them, while the identical varnish goes dead flat when applied through the airbrush.
Thinning the Cabots
Thinning doesn't work very well. I suspect that what it needs is not thinning, but dilution — I may get an un-tinted can and use that to get the exact degree of tint I need. However, there's not much point in that until I run out of Quickshade, and a pot does go quite a long way, especially when it's brushed on.