We've talked in the past about the necessity of thinning your paints. It's of utmost importance to control the consistency and flow of your paint to achieve the best results. The main questions are "what do I use?" and "How thin do they have to be?"
Consistency is sometimes a little arbitrary and often difficult to explain. General rules of thumb are a 1-to-1 ratio of thinner to paint (be sure to do this on a palette!) or the consistency of milk. It's more important to describe it in terms of how it flows from your brush and onto the miniature. This is something that comes from experience and practice. What you want is paint that flows freely from the bristles without needing to drag or scrub the brush onto the surface. In that case, your paint is not thin enough. On the other hand, if the paint pools off your brush and flows uncontrolled onto the model, ruining your work, it's clear that your paint is too thin! It's a fine line between control and easy application - but when you get that consistency right, painting becomes easier and looks better.
What are the benefits of proper consistency? Most importantly, it allows you to get pigment onto the miniature without obscuring the fine detail of the model. A few thin coats will accomplish this better than one thick coat. Also, when your paint flows nicely from the brush, you will not get the streaky surface that shows brush strokes and ruins the illusion of authenticity. It will also allow you the maximum level of control, permitting greater detail, finer lines, better highlights and layering, and a more professional finished product overall.
So, now we have talked generally about "how much" you should thin your paints, and why it's important to achieve the proper consistency. But what should you use?
There are a lot of products available, and much of it is up to individual preference and what else you might be looking for in your paint. We're going to confine our discussion to acrylic paints (because in my opinion, enamels are for amateurs who don't know any better or true professionals who don't need my guidance - there is no middle ground) and the products that work with acrylics.
There are various supplemental products one might add to paint to change how the paint performs. Chiefly among these, let's talk a little about flow improvers and drying retarders, as these are the two things you're most likely to want to change about off-the-shelf paints.
Flow aids work by increasing flow and absorption and decreasing film tension and friction in your paint. In layman's terms, that means the paint leaves your brush more easily (allowing greater control over application) and gets into all the nooks and crannies of the surface you're painting on. In a previous article, we talked about the self-leveling characteristics of AV Surface Primer and Citadel shades. Flow improver basically helps to impart this characteristic to your regular paints, allowing you to achieve more uniform coverage and thinner coats. It also slightly slows the drying time of the paint. However, slower drying time is not the main benefit of the product, and there is in fact a better product made specifically for that purpose.
Drying retarders (sometimes called slow-dry medium) increase the working time of the paint, allowing you more time to blend layers as well as keeping paint from drying on the brush as quickly. This helps you achieve more natural transition between layers, more natural shading and highlights, and incidentally gives you more time to fix those occasional mistakes that require another clean, damp brush to remove paint unintentionally applied where you don't want it!
There are a variety of vendors that sell these products. Hobby paint manufacturers like Reaper and Vallejo sell small bottles of these mediums, but for best value and performance, try a store that sells art supplies and look for products by Liquitex, or Winsor&Newton (yes, the company that makes those great Series 7 brushes!) Instead of paying $3-4 dollars for a half-ounce of the stuff from a hobby line, you can get 4 ounces of professional artist-grade product for about $10. You end up paying 25-30% as much by volume, and a 4-ounce bottle of each will last you a long time. We will see why in a minute.
The aforementioned products do not, in and of themselves, work directly as thinners. Dilution is recommended on the product labels. Flow aids are recommended for use at a 10% ratio, and retarders at no more than 5% of volume. This is because neither product contains a binding medium.
So let's get back to talking about what you might use directly to thin your paints. For a long time, I used the AV acrylic thinner, a milky-white fluid that works fairly well for the purpose. I do not, however, recommend using it with metallic paints and certainly not for washes or shades because it is not transparent when applied (even if it dries that way in normal non-metallic opaque colors).
A better medium to use for thinning paints is Citadel's Lahmian Medium. This is a transparent liquid that is great not only for thinning paints, but also for turning them into washes or shades when used in a greater proportion.
However, both products suffer from the same expense issue as the other mediums in a hobby line - and if you're aiming for a 1-to-1 ratio with paint, you'd eventually go through a dropper bottle of it for each dropper bottle of paint you own. Do you really want to double the cost you're already paying for paint?
Some people have used isopropyl alcohol to thin acrylic paints. I used to do this long ago and I do not recommend it! Unless you are using something like Vallejo's Liquid Metal series or other alcohol-based acrylics (which require different care and techniques), avoid using alcohol to thin acrylic hobby paint. It works by breaking down the binding medium of the paint, which reduces adhesion, durability, and can ultimately ruin your paint.
The least expensive, most common and ultimately best recommended product to use for thinning acrylic paints is good old water. Tap water will do, but if you can, I recommend distilled water. It is free from all of the minerals, trace chemicals and sediment that you may find in your tap water and will not cause your paint to react in unforeseen ways.
Into the water, you will add other fluid mediums as desired, mixing well. I have a large dropper bottle into which I mix the following recipe:
1 part drying retarder
2 parts flow improver
1 part Lahmian Medium
6-10 parts water
"But HeadHunter!" you exclaim, "You said earlier that you should use no more than 5% retarder and 10% flow aid! That formula uses twice as much as the recommended amounts on the label!"
Well, that's true - but that's because of our target 1-to-1 ratio of thinner to paint. Only half of the fluid, of course, is thinner. The other half is the paint you are trying to thin - so 10-20% in the thinner bottle is actually 5-10% total in the thinned paint. Hopefully that clarifies the confusion.
I keep this bottle on hand to thin all my paints to the desired consistency. Until you have sufficient experience, I recommend transferring paint to a palette and thinning it there - but some more experienced painters will thin their paint right in the bottle or pot, once they can eyeball it for the proper consistency (or know what it should sound like when shaken).
I use a Citadel Palette Pad - it's basically the size of a half-sheet of paper and contains 20 sheets of stiff, non-absorbent paper with a glossy finish (you could use both sides if one gets too covered with paint from a series of projects). Transfer a few brush loads of the color you'll be using onto the palette (be careful not to get paint up inside the ferrule!) and then add one small drop of thinner a bit off to the side - not directly onto the paint! Use your brush to draw a bit of the thinner into the pool of paint and mix it around. Draw your brush away from the paint onto a clean spot to draw off excess paint and point your brush. You should now have a moist brush loaded with enough smooth-flowing paint to last a minute or two. No more dipping the brush in the pot every few seconds!
Basically, this recipe costs me less than $20 for about 3 pints worth of thinner - enough to last you a very long time indeed. Give it a try and let me know what you think!