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Was hoping someone who's an expert on the subject could explain the difference, pros, cons etc...

Not an expert, but basically...

Thinning with water works the same as with medium. The difference however is that water changes the consistency of the paint and creates surface tension. Therefore you shouldn't thin washes with water.
Medium "only" introduces more medium and therefore reduces the density of the pigments. Effectively the same with said difference.
Flow-Aid is not used for thinning, but mostly for use with airbrush as it helps "glide" the paint better from the needle, causing less tip dry and therefore smoother workings.

In simple terms, all paint is pigment – ground-up lapis lazuli, for example – combined with a binder. In the case of oil paints the binder is oil, in watercolours it's gum arabic, and in acrylics the binder is an air-drying plastic.

There are lots of materials (generally fluids) that you can combine with a paint, known collectively as 'mediums'. These include mediums to add special effects (such as texture gels, pearlescent medium etc.), as well as those that alter the flow or consistency of the paint. The most obvious of these for acrylics is water, which is used for diluting (or thinning) the paint.

When thinning the paint, you need to use a material that's compatible with the binder – this is why you can't thin oils with water, and why combining white spirit with acrylics will end up a mess.

+++
Water vs medium vs Flow-aid
As the above explains, water, medium and flow-aid are all mediums. However, they have slightly different properties.

Water is a medium that acts to alter the properties of the paint: in this instance, to alter the consistency. Adding water allows the paint to flow further but spreads out the pigment and alters the surface tension, so it works in a slightly different way to 'pure' acrylic paint. We'll all be familiar with this. The downsides of thinning paint with water is that the changes to surface tension can cause the particles of pigment to 'float', instead of remaining spread evenly. When it dries, this can cause patchiness. This will vary from paint to paint, owing to the properties of the pigment – some granulate (heavy particles sink), while other flocculate (the pigment particles clump together). The advantage water has over the others additives is that it evaporates completely, leaving the paint as bright as it can be (assuming you don't have hard water, which can leave tiny deposits of calcium carbonate). It's also cheap!

'Medium' is a catch-all term, but I think the question refers to generic 'acrylic medium', which is sold under various brand names – GW's is called Lahmia Medium, for example, while Winsor & Newton, Liquitex etc. have their own. This is, for our purposes, pure acrylic binder. Think of it as clear acrylic paint and you won't go far wrong. It is added to paint in much the same way as water, but alters the properties differently. Most mediums sold as 'acrylic medium' are intended to duplicate the texture, flow, drying time etc. of the other paints in the range, so by using this in place of water, you are spreading the same amount of pigment out within a larger amount of binder.

These qualities makes acrylic medium useful for glazing techniques (as more of the underlying layers will show through) and has the benefit that the surface tension remains much the same, so it behaves a lot like paint. The downside is that the resulting effect can be slightly meagre or muted, as the binders are not completely transparent and there is less pure pigment by proportion. This is the reason they are often sold as 'matt medium'. With that said, many acrylic mediums – including Lahmia Medium – are slightly less dense than the pure paints in the range, which means that they will feel slightly smoother to paint with; which leads us on to 'Flow-aid'.

Flow enhancers are mediums that are designed to alter the surface tension of the paint. This helps to avoid brushmarks and texture, ensuring the paint flows – hence the name. They are sold under lots of different names – flow enhancer, flow-aid, blending medium etc. This makes them excellent for blending, but they can feel very different to paint with. They vary from manufacturer to manufacturre, but tend to occupy a nice niche between water and acrylic medium; avoiding patchiness by supporting the pigment particles (spreading them more evenly than water) while avoiding the worst dullness of matt medium.

Conclusion
Like all tools and materials, there's no right answer; merely the one that works best with your other equipment and skills. I thoroughly trying all of them out, just to compare and contrast. Note that the surface on which you are painting and the brush you use will also alter the flow and way the paint is deposited – natural hair brushes tend to retain more water in the belly of the brush (good for flow and flooding areas if necessary), while synthetics retain less (good for control).

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The above is a bit of a simplification, but I think it's a good general primer – hope it helps.

 Thairne wrote:
Not an expert, but basically...

Thinning with water works the same as with medium. The difference however is that water changes the consistency of the paint and creates surface tension. Therefore you shouldn't thin washes with water.
Medium "only" introduces more medium and therefore reduces the density of the pigments. Effectively the same with said difference.
Flow-Aid is not used for thinning, but mostly for use with airbrush as it helps "glide" the paint better from the needle, causing less tip dry and therefore smoother workings.

Mediums can be various things, you have mediums that are thick and mediums that are thin. Vallejo for example make a "thinning medium". In general, mediums can refer to a lot of different things. But typically for wargamer type paints they're just diluting the paint with something that will bind with the paint better than water.

Flow improver is what it sounds like, makes the paint flow better, it is commonly used for airbrushing but also for regular hairy brush painting as well. It can be added to thinning mixes for home made washes to make them flow better, when doing fine detail work sometimes it can be beneficial to add a bit of flow improver to help the paint flow off the brush better, sometimes when painting directly in to crevices a bit of flow improver can help as well.

Just to add to some of what other people have said here:

The definition of a medium is the liquid that you can mix pigment with to make paint. One difference between medium and flow enhancers and water is that medium has body. That is, if you put a blob of medium on a piece of plastic and let it dry, it will be thick, whereas if you put clean water onto a piece of plastic, it will all evaporate.

The benefit of medium over water is that if you dilute water-based acrylic paint with too much water, the paint has a tendency to dry unevenly, or worse, to have drying rings, a result of surface tension. Because mediums have body, you don't get drying rings. Mediums are really good for glazing, too.

A flow enhancer is a different beast. It's mostly water (liquitex is literally a concentrate that you dilute with 95%water), often with a soapy addititve, that helps reduce surface tension of chalky paints without adding body. This helps a lot with certain paints, especially whites, to reduce brush strokes. With almost every flow enhancer, the more flow enhancer you use, the weaker the paint is in terms of adhesion. Sometimes, flow enhancers will also have a little drying retarder added into them.

Water is wonderful. 99%+ of what I brush on is thinned with only tap water. It's really only basecoats of yellows and whites that I use much flow retarders for (these can require like, 10+ coats, so each coat needs to be perfectly flat), and glazes, feathering and transparency that I use mediums for.

So since we got some true experts chiming in on this thread - what would you guys recommend when decanting citadel paints into dropper bottles?
One needs something to get more paint out of those terrible pots and rinse it out a tad, prethinning the paint for wet palette use.

Plain old distilled water?
A matte medium (lahmian is out of the picture for obvious cost reasons)?
A Varnish?
Or even.. flow aid?

Here's a good article that talks about a number of additives, glazes, etc

http://archive.brushthralls.com/painting-techniques/additives-glazes-and-smoke-oh-my-2.html

This is really great, thanks to everyone who's posted. I'm curious, as a pot of paint dries and thickens, is it best to add water, or medium, or a mix? Also, any tips for getting no streaks on large surfaces with metallics? Just using liquitex flow-aid seems to help a little, but it's still seems to be luck of the draw to get a coat that looks consistent even after many layers.

 Thairne wrote:
So since we got some true experts chiming in on this thread - what would you guys recommend when decanting citadel paints into dropper bottles?
One needs something to get more paint out of those terrible pots and rinse it out a tad, prethinning the paint for wet palette use.

Plain old distilled water?
A matte medium (lahmian is out of the picture for obvious cost reasons)?
A Varnish?
Or even.. flow aid?

I would use a few drops of this in each bottle- http://www.goldenpaints.com/technicalinfo_openthin

Not to confuse the topic but I use bog standard Humbrol Acrylic thinners. On the lighter colours it works much better than water and metallics do go on more evenly also.

I'm not sure what you mean by "bog standard"

Are you using this stuff?:
http://www.humbrol.com/us-en/acrylic-thinners-125ml-bottle.html

 Thairne wrote:
So since we got some true experts chiming in on this thread - what would you guys recommend when decanting citadel paints into dropper bottles?
One needs something to get more paint out of those terrible pots and rinse it out a tad, prethinning the paint for wet palette use.
Plain old distilled water?
A matte medium (lahmian is out of the picture for obvious cost reasons)?
A Varnish?
Or even.. flow aid?

Depends on what you want to get "out of the dropper".
A small spatula will be in order since the GW paint is pretty much liquid gold in cost: waste none of it.

1) If you plain-old just want the paint to flow out you add as little medium you can until the consistency flows well to decant.

2) I like "ready to paint out of the dropper" so I tend to add 1/4 matt medium, 1/8th water and a drop of flow-aid.
<edit> Note, I found that sometimes adding matt medium to the GW paints is just what they need, it may be why they dry out so much.

3) For "ready to airbrush out of the dropper" I go 1/2-3/4 airbrush medium for Liquitex medium body paint or about 1/4 for the runnier paints. (Or, still fussing with this: 1/4 isopropyl alcohol and about a drop of flow-aid for every 1/4 oz). NOTE: This is all dependent on needle size and pressure you are running at.

4) For "shade" it is really paint made very runny and with a high amount of flow-aid. I would suggest trying acrylic inks like with Liquitex, nail it with some flow aid and it makes a fine shade that wicks into recesses. I have been experimenting with alcohol as an additive to see if it helps prevents the rings discussed earlier.

NOTE: your mixes are based on preference of how much more or little drying time you want.

I keep hammering on the Liquitex since I get a good volume of it and it is really high pigment so I do not get any surprises.

My suggestion is to buy both medium and flo-aid (the latter is super cheap), and experiment with both, as compared to water. They are different and equally useful.

Medium has body, whereas flo-aid is practically all water -- meaning that once it evaporates, medium adds thickness whereas flo-aid adds almost none. Just put a drop onto a non-absorbant surface (like a palette), let it dry, and you'll see what I mean.

Medium is most useful for glazing and thinning a paint such that it has the same viscosity, but has less opaqueness (or more transparency). This prevents thin paint (where the ratio of thinning liquid to paint is high) from drying with ringlets, since the pigments are suspended in medium that will dry in-place. Think of it like transparent paint. In contrast, if you have paint thinned with water, as the water evaporates, what's left pulls together (surface tension), leaving drying rings.

Flo-Aid is a soapy liquid diluted with lots of water (liquitex is 20:1 water) that helps make the paint pigments spread out and diffuse of the area that you paint. This is most useful if you have chalky paints that tend to dry with brushstrokes that you want to spread over a large, flat area (and not have brushstrokes).

I'm sure you know what working with water is like Frankly, for the vast majority of my painting (95%+), water is my preferred thinner. It's plentiful and free, works great, and is very consistent. I do use mediums extensively for glazing, and flo-aid for basecoats on tough colors, like yellow.

Talizvar wrote:
2) I like "ready to paint out of the dropper" so I tend to add 1/4 matt medium, 1/8th water and a drop of flow-aid.
<edit> Note, I found that sometimes adding matt medium to the GW paints is just what they need, it may be why they dry out so much.

Thanks a lot, this is exactly what I want to go for.
I already DID decant all my citadels using, in my... erh... limited experience with that kind of stuff, airbrush thinner. Well, worked mostly, but sometimes I used just too much and the paint became TOO thin for basecoating. Was a bit runny.
So, with 12ml paint, that would equate to 3ml medium and about 1-2ml water with a drop of flow-aid.

After decanting thinning further for airbrush use is a breeze. Made airbrushing a joy instead of a struggle.

@Talys
I have everything available in my paint drawer
Vallejo Flo-Aid, Airbrush Thinner, Liquitex Matte, 5 litres of distilled water... what have you.
Just trying to figure out what to use to a) prethin the paint for out-of-the-dropper-painting and b) get the remaining paint out of the pots. Some basecolours are so thick that they actually clog the funnel I use, preventing air from escaping and therefore hanging in a limbo. Having to push the stuff down with a thin wooden stick is even more paint wasted.

I thin with Future. It's incredible. Its a clear acrylic flow improver I think.

I'm pretty sure I've read a few places that Future changed their formulation, and can now can cause some pretty bad discoloration of the paint. Maybe they're still making it the old way in Japan though.

 Weboflies wrote:
I'm pretty sure I've read a few places that Future changed their formulation, and can now can cause some pretty bad discoloration of the paint. Maybe they're still making it the old way in Japan though.

It used to be called Klear. I have some of the "new" Pledge Future stuff that doesn't seem, to discolor, but I never use it anymore -- I'm not 100% sure if the formulation has changed, even though it seems to work the same. I used to use Klear for like, everything.




Automatically Appended Next Post:

AesSedai wrote:I thin with Future. It's incredible. Its a clear acrylic flow improver I think.

Klear/Future was just colorless acrylic resin, I believe (used to polish floors).

Thairne wrote:@Talys
I have everything available in my paint drawer
Vallejo Flo-Aid, Airbrush Thinner, Liquitex Matte, 5 litres of distilled water... what have you.
Just trying to figure out what to use to a) prethin the paint for out-of-the-dropper-painting and b) get the remaining paint out of the pots. Some basecolours are so thick that they actually clog the funnel I use, preventing air from escaping and therefore hanging in a limbo. Having to push the stuff down with a thin wooden stick is even more paint wasted.

Hah! That's cool By the way, I prefer Liquitex flo-aid to Vallejo by a long shot, and it costs a tiny fraction, too.

I have not had a lot of luck pre-thinning with flo-aid. I did this for a short time, a long time ago -- I would keep a pot of prethinned and a pot of regular paint. The problem for me was that the flo-aid prethinned stuff would dry... strangely after a few months, and invariably, I'd have to stir it with a stick because it would separate more than the other stuff. If you use GW paints, the amount you need to prethin it whether it's a base or layer paint, and the color varies, which makes it a pain. With Vallejo, be mindful also that the amount of thinner will also vary depending on the color. For example, don't thin yellow at all.

Also: I wouldn't add liquitex matte to Vallejo paint as a pre-thinner, unless you do it for every paint. It makes that paint much, much more matte, and your finished model will look funny if you don't varnish it

If you just want less opaque paint, I think a semi-gloss medium is the best alternative. If you want thinner paint for airbrush, I like Vallejo airbrush thinner, a buddy of mine likes to add a drop of Vallejo airbrush flow enhancer.

If you use an airbrush and overthin your paint, it will tend to spray out in a big puffy cloud and kind of be hard to control, and also run a lot (corrected by using less PSI).

 Talys wrote:
I think a semi-gloss medium is the best alternative

Is there one you can recommend?

@weboflies -

If you're using GW paints, Lahmian medium has the same glossiness as GW paints, so that's a plus

I believe Vallejo's Glaze Medium is a semi-gloss medium, too. I can't recall what Liquitex's glazing medium looks like when dry. But Liquitex matte, ultramatte, and gloss mediums definitely look nothing like Vallejo and GW paints out of the pot.

Oddly, I find that Vallejo Decal Medium has a consistency that's very similar to Vallejo and GW paints. I think it's much nicer a finish than MicroSol, which dries super glossy. However, I wouldn't recommend it as a paint medium for sure, since it's not water based (it's alcohol based), and it goes on goopy.