There are a number of acrylic mediums produced for using with decorative painting
to get particular paint effects. For instance, cracking medium is used to deliberately cracking a varnish or layer of paint to get an aged effect. Marbling medium is used to stop paints mixing together, producing swirls of unmixed color. Antiquing medium is used to make something look old and worn at the edges.
To get the best results with these types of acrylic mediums, take the time to read the instructions carefully before you start. With some it's just a matter of adding it to the paint, but with others there's a sequence you need to follow to get decent results.