One of the techniques I love most in painting, but which takes a lot of time, because you have to wait for each layer to dry before you add the next, is glazing. You can glaze well with oils, but I mostly work with acrylics and the technique produces stunning results.
When you apply glazes, you get deep, realistic colours, which are vibrant and alive. Somehow the colours mix like optically perfect stained glass, rather than as a colourful mud. You can be ever so daring with colours that would normally mix poorly when wet. As dry glazes, the colour effects and combinations are pretty much unreproducible, if painting alla prima. You also get a tonal balance and a colour unity to your painting that is unobtainable using other techniques. It’s one of the easiest ways of adding shadows to a painting as well!
Leonardo Da Vinci used 30 layers of glaze to produce the Mona Lisa, they are now discovering, using the latest X-ray fluorescence spectroscopy techniques, yet the finished thickness of the paint was less than that of a human hair. “Smoke” (sfumato) indeed! http://www.aolnews.com/world/article/scientists-unlock-mystery-of-mona-lisa/19558222
The idea is to paint a black and white tonal background (grisaille) first, then build up the colour like they used to tint old black and white photographs in the 1940s, with coloured inks, using an air brush. With glazing, though, you use a real bristle brush, not an air brush, though I bet you could use the latter, if you thinned the glaze far enough.
To make your glazes, you mix 90% acrylic glazing medium (there is a choice of drying times available) with 10% paint. The idea is to get paint that looks like a very thin ink in its colour density (though the viscosity need not be so thin), so that it dries like coloured cellophane on your ground (canvas, in my case).
Here is an excellent tutorial on the glazing technique, using oils: http://www.wetcanvas.com/Articles2/32418/530/index.php
These are the mediums I use: http://www.chromaonline.com/products/atelier_interactive/mediums_grounds
Some good videos on how to use these unique acrylic mediums are on this page: http://www.chromaonline.com/media_library/multimedia/atelier_interactive_mediums
Another brilliant range of mediums I happen to like are these: http://www.goldenpaints.com/justpaint/jp8article2.php . They dry very clear and transparent, with no yellowing or cloudiness.
Now, you don’t even have to mix your own glazes. You can buy them pre-mixed (I haven’t tried these, but want to) http://www.goldenpaints.com/products/color/glazes/glaze.php . These are ready to go, straight from the bottle!
Another glaze I intend to try is the new optically-transparent medium from Winsor and Newton: http://www.winsornewton.com/products/oils-solvents-mediums–varnishes/acrylic-colour—painting-mediums-texture-gels–varnishes/artists-acrylic-mediums/acrylic-mediums/glazing-medium/. They claim “maximum transparency” and that is what you want.
Give it a shot. It takes time, but the results are simply wonderful. Happy glazing!