If you are painting something and you want to give the paint an antiqued looking, with cracking and old colors showing through, this effect can be easily obtained using fish glue (a protein-based glue which usually comes in a can like paint). To get the effect, you'll need two different colors of paint. One to act as a base coat (which will show through the cracks), and one to act as a top coat (this is the one that will be cracked). For the base coat, you can use either a latex or oil paint. But to get the best effect, always use a latex paint for the top coat.

Once you have your paints, apply the base coat and allow it to dry completely as per the instructions. Once it is dry, take the fish glue and dilute it with water by 20%... 4 parts fish glue to 1 part water. Then apply the diluted fish glue over the base coat with a brush. Allow the fish glue to dry until it is tacky (it'll be sticky, but it'll stay in place and it is *not* completely dry). Once the fish glue is tacky, apply the top coat. You need to apply the top coat when the fish glue is tacky to get the cracking effect. The tackier the glue, the more cracking you will get in the top coat.