What is the primary reason for mixing a thinner into paint?

Mixing a thinner into paint primarily serves to spread the paint easily. Thinners are solvents that reduce the viscosity of the paint, making it more fluid and easier to apply. When the paint is thinner, it flows better, allowing for a smoother application, whether through brushing, rolling, or spraying. This increased fluidity helps to achieve a more uniform coverage on surfaces and enhances the overall finish of the paint job.

While other options might also relate to paint application, they do not capture the primary function of a thinner. For instance, clearing up air bubbles can occur during application, but it is not the main purpose of adding a thinner. Similarly, thinners are not designed to prevent bleeding of paint or to lighten the color; their role is primarily centered around improving the paint's workability and application characteristics.