Fixture of Dakka
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I keep 3 types in little bottles --
Liquitex Flo-Aid, Vallejo Airbrush Thinners, and various acrylic Mediums (Vallejo, Citadel, and others). In addition, there are drying retarders.
Flo-Aid - This slightly slows drying time, but not by much. For me, its most important function is that paint thinned with flo-aid dries more brushstroke free (as Kr00gz states accurately, this is because of reduced surface tension). It comes in a concentrate, and is diluted in water (10:1 to 20:1), and therefore a small bottle goes a really long way. Underdiluting Flo-Aid makes your paint not stick to surfaces very well.
Flo-Aid is very helpful thinning with certain colors such as white or bone, which tend to dry in a very chalky way that shows every brushstroke. It is less helpful (but not harmful) thinning colors like blue or black that largely dry nicely.
Vallejo Airbrush Thinner - I keep this to dilute paints not made for an airbrush to spray, most notably, Citadel paints (I just use the Air versions of the Vallejo). I also use it to dilute Vallejo air paints for 0.25 and smaller needles. I don't know what the technical description of Vallejo Airbrush Thinner is, but for me, it serves two purposes: first, it significantly increases drying time, which is REALLY important in an airbrush. Second, it makes the paint thinner so that it goes through an airbrush nozzle. By thinner, I mean, with 1 coat, it will look less opaque and more transparent, and it will run more. I do not think this is ideal for paintbrush.
Acrylic Mediums, including the various Vallejo ones, Liquitex mediums, and Citadel Lahmian Medium, are basically paint pots without the pigment. So, for instance, Lahmian Medium is Citadel paint, with no Citadel pigments. This is really handy for many things. If you use a little bit, the paint will be more runny and tend to dry a little more slowly. You won't change the tint of the color, but the opacity will be slightly reduced. This is useful for doing things like writing text, because you determine how thick the paint is without affecting any other property.
You can also use medium in more generous amounts to create a glaze. Basically, a little bit of pigment in a lot of medium gives you a glaze (or wash). It's particularly useful, if for instance, you want to glaze white with pink (as nobody makes a pink glaze). Because you control how much pigment is in the medium, you can balance the ability (or lack thereof) to cover with the runniness.
Vallejo (and Liquitex, and Golden) has different mediums that give you a different finish. You'll notice that some paint brands are more glossy, matte, or in-between (satin) -- this allows you to adjust that. Not really a big deal if you're going to varnish, but more important for photography types that want to take a picture after a model is painted and before it's varnished.
Also, a trick that some people use to affix decals is to use a glossy medium first (and let it dry), in order to give a decal a better surface to glide on. This happens because the glossy medium has less pits in it and is more friction free. After the decal is affixed, they paint a thin coat of flatter medium on top.
Finally, there are drying retarders and "blending aids". Basically, the dry time is very significantly increased. You can get it in a very liquidy format, or a gel-like medium. With some of the stuff, if you use enough, a pool of paint in a palette will stay wet for *days*.
Drying retarders are helpful if you're doing things like writing text, because not much paint goes onto a brush, so it may dry before you put the brush onto the work surface. When doing wet-on-wet blending, or feathering, a drying retarder may help, especially if you're trying to learn the techniques, because there is a tendency for some brands of acrylics to partly or fully dry before you have the second brand on.
On the other hand, as you become more experienced with wet blending or feathering, it becomes a bit of a nuisance, because paint stays wet after you're done with an area, and you have to give it more time to dry before you can work near that area again.
Personally, I use Flo-Aid a lot (especially when I paint near-white/bone colors), airbrush thinner whenever I use non-airbrush paint in an airbrush, medium sparsely, and drying retarder almost never.
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