Key Points

a. Acrylic is the most versatile of all painting mediums.
b. For the beginner acrylic paint can radically simplify the many exasperating technical problems that other painting medium present, and to the more competent artist it opens new fields.
c. Acrylic has gained an unfair reputation as having an advantage only when used in flat areas of bright color.

What are Acrylics?

Acrylic paint is fast drying paint containing pigment suspension in acrylic polymer emulsion. Acrylic paints can be diluted with water, but become water-resistant when dry. Depending on how much the paint is diluted (with water) or modified with acrylic gels, media, or pastes, the finished acrylic painting can resemble a watercolor or an oil painting, or have its own unique characteristics not attainable with other media.

http://en.wikipedia.org/wiki/Acrylic_paint

Why choose Acrylic?

When choosing paint for a miniatures project you must consider a number of points.

Will the object be handled? If so, it should be painted with enamel/oil paint. Acrylic paint will need special sealers to allow repeated handling.
Is the object porous, does air get through? Acrylic paint works best where air passes freely through the object. On plastics or metals, acrylic paint will only survive if it is painted on top of a sealing base coat.
Which paint suits your working style? Oil/enamel paints take longer to dry and allow more time to adjust colors. Acrylic paints are fast drying and easy to clean up after.

Easy Clean Up

Acrylic paints are easy to clean up using soap and water. Only mix small amounts of paint at a time, use a flower palette (see basic tools) with a cover, and cover unused paint when you leave your work. Paint stored under a cover will last for up to 24 hours before drying out. The time depends on heat and humidity levels in your work area. Dried bits of paint are easily peeled off the painting palette when dry.

Best Types

Acrylic paints come in a wide variety of forms. If you haven't used them before, learn to blend your own colors using artist’s quality acrylic paints and acrylic mediums, rather than using hobby or craft paints. Artists quality paints are much better colors with much less filler than student quality paints.

Student quality is usually above the standard of craft paints. Craft paints are usually more opaque with more fillers than artist’s paints. Craft paints are not rated for lightfastness or pigment content.

Many craft paints are specified in particular instructions. If you want your miniature to match, use these.

Acrylic Mediums Enhance Effects

Learn to use acrylic mediums, extenders and thinners, to create the effect you want. A wide variety of acrylic mediums are available which extend the handling qualities of the paint. Some thin it, some add texture, some change transparency, some even allow you to use the paint as a fabric paint. Sample jars of mediums are available. Using Pumice medium will allow you to instantly create a stucco texture.

Best Used on Porous Materials

Use acrylic paints on materials which breathe and do not trap moisture – paper, wood, terracotta, bisque.

If you will use acrylic paints on non porous materials: metal, plastic or resin, use proper undercoats and overcoats. These materials do not breathe so anything painted on them needs to dry perfectly and not swell or contract. If possible, use enamel paints on these materials, or use undercoats with acrylic paint to help the acrylic stick properly then seal the acrylic coating with an overcoat to prevent it absorbing moisture.

Fast Drying Thin Layers

Acrylic paints are fast drying and can be thinned with water and acrylic medium to apply very fine layers.

Use thin coats to accent surface detail. Thick coats of paint will fill in the details and lower your opportunities to add highlighting washes.

Acrylic paints are idea for situations where you want to apply washes to highlight miniature details. Very thin yet opaque coats can be used to create the base, then detail washes can be applied over the base coats without any danger of blending or bleeding.

Do not over thin acrylic paints with water, the paint will become weak. Use a mix of water and acrylic medium.

Soft Coatings

Acrylic paints never completely dry out. They are hydroscopic and will swell slightly with moisture. They are not for use where wear is important. If you need a hard lustrous coating use oil/enamel paints.

http://miniatures.about.com/od/miniatureprojects/bb/acrypaintstart.htm