I will leave the older web-based article below, but here is the print version, a bit better layout wise -
Ok, the little crash course that was requested in mediums/fluids -
1. Glazing medium
All acrylics are of course designed to be thinned with water. Many people glaze perfectly well using only water as the thinner. At a certain point though, adding too much water thins the amount of suspension/binding medium that is an inherent part of the paint, and it loses it's consistency and ability to stick. You've seen it happen, it is that point where so much water is in the paint that it doesn't leave brush strokes anymore, it leaves a trail of beaded droplets and 'bald spots'. Glazing medium is a way to maintain the integrity of the paint while still thinning the pigment count, it is essentially thinned suspension medium, designed to maintain all the qualities of the paint while modifying it's transparency.
This is where the term 'glazing' comes in. In essence you are dealing with a thicker wash. With 1 critical difference though.... Washes, such as GW's, are primarily designed to pool in recesses, Glazing is in effect applying the paint via traditional, controlled brushwork, not just blobbing an entire area. You build the color by layering the now transparent paints over and over until you achieve a flawless blend, modifying the colors tiny step by tiny step. Think of it as turning off the lights by putting on sunglasses, 1 pair will only darken your view a touch. Glazing is the art of putting on sunglasses over sunglasses over sunglasses until you get to black (or whatever color you were looking for!). Besides the way you apply it being different, a wash tends to be a lower viscosity fluid to facilitate it easily and automatically getting into the cracks and recesses. Generally, this is done via the introduction of a flow aid in addition to glaze medium, and most flow aids make the end result slightly (or very) glossy. Glazes should dry matte. Glazing works particularly well over white, and the two together can achieve very vibrant yet still natural results.
Most of the time, the best skin you will see on mini's is the result of glazing -
A more comprehensive tutorial from the amazing team at Massive Voodoo-
Things to note: As is seen in the Massive Voodoo article, it is not always neccesary to use the medium to achieve a glaze. However, use of glazing medium is a small step that can be taken to ensure that you will not over-thin and ruin your glaze. To me, this can be particularly important to get that guarantee, as glazing can be unforgiving due to it's transparent nature. If you mess up badly, you will not have a choice but to use full opacity paint to cover it up, at which point you have ruined the luminosity and blending of the glaze. It can be a bit unforgiving until you get a feel for it.
Recommended glazing mediums from me -
Plain old water - just be careful and don't thin too much.
2. Flow aid (dispersant medium)
Flow aid is pretty self explanatory. It pretty much applies to any additive that lowers the viscosity/surface tension of paint. What does that mean? It means that the paint flows better, that it lies flatter to the surface and in many cases is generally easier to work with.
What is viscosity? The answer is rather complex actually, having to do with some pretty advanced fluid dynamics http://en.wikipedia.org/wiki/Viscosity - But for our purposes, let's just say viscosity is a question - 'How much does this fluid stick to the side of a glass?'. Picture putting, say, honey in a cup. Swirl it around, or try to. It does not flow easily, it sticks to the sides etc etc. This is high viscosity. For low viscosity, think of something like vodka, or future floor polish. It flows like grease on ice, leaves almost no residue on the sides of the cup. In essence, we are talking about liquid friction, a fluid's ability to stick to itself and other things, how easily a droplet can be released from the main body.
So, basically, flow aid turns your half and half cream paint to vodka paint, or somewhere in between.
Recommended Flow aids from me -
LIquitex dispersant medium,
Golden dispersant medium,
future floor polish,
some dishwasher detergents/drying liquids will work if you are in a pinch.
Retarding Medium (Slow dry)
Again, pretty self-explanatory. Adding this to your paint will slow it's drying time by quite a bit, giving it an oil like quality for a while (how long depends on how much retarder you add, but in my experience, you shouldn't add too much. experiment a bit, each type of paint will react a little differently, but expect/aim for about an hour). Why do it? If wet blending is your thing, using retarder is almost mandatory. For those who don't know, wet blending is a technique involving putting 2 blobs of different colored paint beside each other on the model itself, and then blending them into each other with the brush right then and there! Like a well applied glazing session, it allows you to reach an essentially seamless transition between colors. It can be particularly helpful with NMM on large areas. Note, if you try wet blending, make sure to read the Brush Brothers tutorial and remember, light to dark!
Thinners, as far as I can tell, are a bit vague. What a thinner is seems kind of contextual to what you are doing. For an airbrush, a thinner is something like a brand thinner, isopropyl alcohol, windex or even plain old water. Thinning is usually mandatory with an airbrush in order to get it to function properly, as the tiny nozzle needs as thin and low viscosity fluid as you can manage without compromising the cohesion of the paint in order spray right. It is tricky though, as you need to make sure, just like with glazing, that you don't overdo it or the paint will not adhere properly on the model and patch up. With brush painting, it almost applies to anything that could be considered a dilutant... water, flow-aid... In general, you will probably end up with a flow-aid type fluid if you buy 'model thinner', whereas you will get a diluted alchohol type fluid if you buy airbrush thinner.
I am not entirely sure of this particular category though, I am basing this on my own observations and what I could find online, if someone has something more concrete to add though, please do!
Matte, Satin and Gloss Mediums
We are all familiar with these, at least in terms of sealers for our finished work. But each has some interesting applications for the actual painting process itself as well. An example being temporarily putting a gloss finish on a model in order to lower viscosity even more and make washes flow like butter in a hot pan (they will find and sink into recesses even more effectively than normal). Gloss sealer also facilitates better contact between a decal and a surface, making it flatten and adhere better than normal (aided even more by decal fluids, but that is another bag of beans).
Matte, gloss and satin mediums were not actually created expressly as sealers, that is a secondary function particular to our type of art in particular, although painters do sometimes employ them to modify or protect their canvases as well. These mediums are, in actual fact, meant to be used to modify the qualities of paint colors! Most painters with a bit of experience will note that certain colors, and certain types of liquids like washes, exhibit their own finish qualities. Vallejo Game Color black is a bit glossy. Washes and inks are almost universally glossy (usually as a consequence of the flow-aids they incorporate). Other paint colors and brands dry extremely Matte, like Citadel Foundation paints.
So, what do you do do if you want your black shiny like a patent leather? What do you do if you want your black flat like soot? Pretty simple answer, simply add a drop or 2 of the appropriate medium/varnish/sealer (whatever your brand calls it) and voila, your color will be modified with the reflective qualities you were looking for. For instance, I often mix gloss into the black of my base rims. I often seal minis with a mix of matte and satin. If you are painting blood, a lot 50.50 mix or dark red and gloss will look nice and fresh. If you are painting a cotton cloth type material, a drop of matte in the mix will ensure it looks right. Simply bear in mind the reflective qualities of the material you are trying to simulate via painting, and use the mediums to ensure your paint will work with you and not against you!
For those who want to know the science of how these liquids create the effects discussed, it is actually quite simple. If you were to take a microscope and look at a dry matte medium surface, you would note that it is quite grainy, bouncing the light in all directions over it's minutely rough surface. It diffuses the light. With Gloss, if you were to look, you would see that when dry the surface remains quite smooth and flat, like a mirror or a sheet of glass, which of course gives it a reflective quality. Satin sits somewhere in between. So basically, they are all different types of surfaces that act with similar qualities to frosted, rippled and clear glass.
Recap: What are they for again?
Glazing medium - Used to create translucent, but very controllable paint. Used to build up smooth, bright transitions, especially vibrant when used over a white basecoat. Glazing paint should behave the same as normal paint (after you of course thin the normal paint with a bit of water as per usual)
Dispersant medium - Generally a good idea in almost any paint style. Makes the paint flow and cover better, reduces any lumpy brush strokes, makes the paint settle closer to the surface. The only time I can think of that you may not want to use it is for drybrushing, but even then, wouldn't matter. It is also an important ingredient for washes and inks.
Retarding medium - Very helpful for those who employ advanced blending, gives you more time to perfect your work. Not so helpful for fine line details or edging.
Thinner - Function and use contextual, generally will do what it sounds like it will do though.
Matte/Satin/Gloss Medium/Sealer/Varnish - Can be used to modify the finish of dried paint, can be used to protect work after completion, can be used to enhance the effects of viscosity based capillary action, can be used to enhance contact between surfaces for things like decals and crazy glue.
Bonus content: Suspension/Binder medium
This stuff is the core of acrylic paint... it is the polymer resin emulsion that allows the pigments to float without pooling at the bottom (mostly) and gives the paint it's plastic like quality when it is dry. If you want to make your own paint, all you need is fine ground pigments, binder medium, flow aid and distilled water, and boom, you have your own custom colors. Not everyone will need this, but it is very cool if you are adventurous. It is also useful if you want to modify a paint you already have and thicken it.
A fine example of using acrylic binders and pigments -
And there you go boys, a small crash course in mediums. You will find the more you learn, the more you will realize that all acrylics are essentially usable for mini paintings, some art or craft paints being superior for certain colors even! It is all simply a matter of pigment count/grind and understanding how to modify the paint using the above to a consistency you like for mini paintings. The point is, as long as the pigments are good, you can turn any paint into mini paint, so experiment with anything you can get your hands on! Don't forget either that these mediums apply to any type of acrylic painting, from miniatures to canvas. Also, the terminology used, like viscosity and glazing also apply to oil paints as well.
If you want to expand your knowledge, start at Massive Voodoo and Brush Brothers, both very talented and generous with their knowledge... there are a ton more, but those are my favorites.