The mineral spirit is a critical component in the process of thinning paint. It does two things: it helps the paint flow more quickly and dilutes the paint, so it's not as thick or dense. The latter function will depend on how much you want to cut your color, but if you're starting with painting, you'll want to use a medium amount of mineral spirit - about one-third cup for every quart of enamel paint. That way, your colors will be less likely to clog up your brush or drip off your project before they dry correctly.