Most people know what a wash is. Mix up a very thin solution of a suitable dark colour (often, but not always, black) thinned down with whatever suits the paint you use (water in the case of inks or acrylics) and brush this over all or part of your model. The wash collects in all of the little cracks and crevices of your model and provides a very effective shadow.

I've come across an enhancement to this technique that works very well if you use water based paints or inks. I very much doubt that it would work with enamels or other spirit based pigments, although there is no reason why a water based wash couldn't be applied to a model previously painted with enamels.

The trick is to thin your ink or paint down with (wait for it) floor polish. Yes, that's right, floor polish. The original write-ups that I've seen for the technique quoted "FUTURE acrylic floor finish for non-wax and regular floors" by Johnson & Son. This is a US product, but I use "Johnson Klear" which can be bought from supermarkets in the UK and this seems to be the same thing. It's an almost clear liquid, slightly thicker than water. It's also very inexpensive, I bought a bottle of around a litre-and-a-half for £3.79 and I've hardly touched it in a couple of years.

Mix one part of acrylic finish to four parts of water. Making it up ahead of when you need it allows the small bubbles which can result when mixing with water to dissipate. Make up a litre or so and store it in a (sealed) bottle or jar and you'll always have it to hand. When you are ready to apply a wash, mix a little of the Magic Wash stock with a small amount of your chosen pigment. With inks, we're talking a few drops in a jar of wash solution (I keep a spare GW paint pot of black wash to hand, since it's the most commonly used wash colour). Inks probably work better than paints, either specialist ones such as GW's inks or paints, Tamiya clear colours or ordinary inks (Winsor and Newton spring to mind) from art shops. Ordinary india ink is fine for black.

The effect of adding the floor polish is to make the pigment of the wash suck down into all of the nooks and crannies and makes the wash less patchy - don't ask me how, something to do with surface tension I guess. Black or brown washes look especially good on Aeronef decks to bring out the planking detail. It also makes a decent shadow around access panels and windows. The only downside to the technique is that it makes your models a bit shiny. But then you're going to varnish them anyway, aren't you ?

As I've mentioned, I'm not claiming to have discovered this idea, I've seen it on several mailing lists and websites so I can't credit the original discoverer of the technique. It's certainly worth passing on.

UPDATE - I've discovered a very comprehensive summary of the uses of floor polish here.