Acrylic paint is fast-drying paint containing pigment
suspended in an 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), the finished acrylic
painting can resemble a watercolor or an oil painting.
Acrylics are sometimes used in place of watercolors
because acrylics dry closer to the desired color (slightly darker,
usually) while watercolors dry lighter (and often unpredictably,
especially for beginning artists).
Acrylics are often used as an alternative to oil
paints because acrylics dry much faster. Oil paints, which consist
of pigment suspended in an oil (usually linseed, or other natural
oil) base, can take a very long time to dry. Acrylic paints can
achieve an oil-paint-like effect, and do so in much less time. Though
applied to look like oil paints, acrylics are somewhat limited due
to the superior color range of oil paints, and the fact that acrylic
paints dry to a shiny, smooth (some say 'cartoonish') effect—not
surprising since acrylic paints are, basically, plastic. Accordingly,
acrylic paint can be removed with turpentine, mineral spirits (also
known as white spirits), ammonia, or rubbing alcohol.
Acrylic painters modify the appearance, hardness,
flexibility, texture, and other characteristics of the paint surface
using acrylic mediums. Watercolor and oil painters also use mediums,
but the range of acrylic mediums is much greater. Acrylics have
the ability to bond to many different surfaces, and mediums can
be used to adjust their binding characteristics. Mediums can change
the sheen from gloss to matte, or can add iridescence or texture
to the surface. They can also be used to build thick layers of paint:
gel and molding paste mediums are sometimes used to create paintings
with relief features that are literally sculptural.
Acrylic paintings, ideally, should be treated as
if they're as different from oil paintings as are watercolors: they
are their own artform. There are techniques which are available
only to acrylic painters, as well as restrictions unique to acrylic
painting. Therefore, judging an acrylic painting as though it were
an oil painting (or a watercolor) is not always appropriate.