Paint is any pigmented liquid, liquefiable, or mastic composition that, after application to a substrate in a thin layer, converts to a solid film
Ever wondered exactly what paint is made of - what distinguishes a gloss red acrylic paint from a matt blue enamel? Despite all the complex chemistry at work in mixed paints, the overall recipe for paint is surprisingly simple and uniform across the board. Here are the four components that make up any paint, modeling or otherwise.
Perhaps the most important part of a paint is the pigment, this is what gives the paint its color. Pigments are ground up powders of a coloured solid material, often metallic compounds such as Titanium Dioxide (white), Iron Oxide (red) and Phthalo Blue. Some colored pigments are naturally occuring and some are man-made but they all achieve the same purpose - to reflect colored light back when exposed to white light. Humans have been mixing pigments such as these to make paintings for thousands of years and the range of colors available to paint manufacturers today is fairly comprehensive.
Although powedered pigment can be applied to a model directly (some people do this for weathering effects), you really want your pigment to create a nice uniform coating. This is where the binder comes in. Binder acts like a glue to stick the pigment to the model in a tough and even layer and is made from a synthetic or natural resin. A popular binder used in model paints is acrylic which gives its name the family of paint.
The solvent is the third key ingredient in paint and acts as the dilutent and thinning agent, keeping the paint liquid until exposed to air. The drying process of paint is the the solvent evaporating and leaving the hard binder + pigment mixture behind to create a paint coat. Extra solvent can be added to a paint to "thin" it down and create an easier mixture for application. The solvent in model paint will either be water, alcohol, or an organic solvent designed to replicate the properties of turpentine. If a pot of paint is left for a period of time the solvent will often rise to the top, this is the watery liquid in the top of the pot when you open a brand new paint and this is why it is so important to always mix your paints before use
Besides the three main categories of ingredients, model paint will have a wide variety of additives designed to alter properties of the overall mixture. One common additive is a matting agent that gives the paint the flat (matt) finish that the majority of models require.