Most Acrylic paint does not need to be mixed and thinned. However, most enamels do.
This guide will use Humbrol enamel paint as an example. The principles are the same for all brands and types.
What you need:
- Airbrush Paint
- Thinner (of the same paint type)
- Stirring Tool
Open up your chosen paint container/can and dump out the contents into an airbrush jar. Nine times out of ten the paint in the can is very thick and needs to be scooped out. Get out what you can and put it into a jar. Now would also be a good time to label your jar with the color, type, and brand of paint. I've got about 30 jars, some with varying shades of the same color. Without labels it would be hard to remember exactly what is in which jar.
Your mixture should be 2 parts paint and 1 part thinner. This can however be difficult to gauge at first as the paint that comes out of the cans vary in thickness. The goal thickness is the consistency of milk. Too thin and the paint won't cover well and will run. Too thick and nothing will come out, or it will come out in large droplets.
A humbrol can is 14ml so in a perfect world I would need 7ml of thinner. This is not always the case as some paint is already fairly thin. Below is a picture of the paint straight from the can. The solid blue at the bottom is mostly paint where the more clear looking liquid is the thinner that Humbrol put in the can at the factory. This is why you can't put 7ml of thinner in each Humbrol can.
When I thin my paint I fill a syringe with thinner and put it in the paint 1-2ml at a time. You can always make paint thinner, but not thicker. After each dosage of thinner I mix the paint with my mixing tool, making sure it is evenly mixed. Getting a feel for paint that is too thick or too thin takes time. I've found a good way to tell the thickness based on how the mixer looks when it is pulled out of the paint.
To use the mixer as a gauge first start by turning off the mixer while it is in the paint and then lifting it out. Take note of how much paint is on the end. Next put the mixer back in and turn it on. Pull it back out and as you are doing so turn it off (time it right or you'll have paint everywhere). Take note of the thickness.
In the above image the black is too thin, the blue is just right, and the yellow is too thick. In the thin paint the black is not adhering to the ends of the mixer. This gives it a watery transparent look. The yellow paint is so thick it clumps together into a bump on the end of the mixing stick. The blue is just right, it covers the mixing stick evenly and does not have excess paint clumped on. When mixing the paint take note of how many bubbles there are. The more bubbles, the thinner the paint. Paint of the correct density should have little to no bubbles.
If you accidentally mix paint to be too thin you have three options; throw it out, mix in a new can of un-touched paint, let it sit out for a few days - stirring it every other day - so the thinner can evaporate.
Paints of the same type (and preferably same brand) can be mixed together to create your own custom colors.
If you are coming back to pre-mixed and/or pre-thinned paint, and you have properly stored it with the holes covered, all you have to do is put your mixer tool in and turn it on. The paint will have clumped together at the bottom so be sure the paint becomes an even consistency once again. Below are two paints that have been sitting for about 3 days. Notice they have already begun to separate and clump together, so don't forget to stir even if you just used the paint yesterday.
Thinned and ready to go;