Solvent-based paints expire after about 15 years, and latex-based paints last about 10 years. These time frames assume that the paint has not been allowed to freeze, according to BobVila.com.
A skin sometimes forms on the surface of older paint, but this is easy to remove and does not indicate that the paint is bad. However, to be viable, the paint must be free of lumps and easy to stir. Smelling the paint is another way to determine if it is still good, according to Capstone Painting. Water that mixes in the paint from brushes or rollers can cause mildew to grow and contaminate the paint beyond use.