There are so many different thinning/reducing products on the market today that it can be tough to know which one is right for the task. The best option is the one specified by the manufacturer for the specified application method. Not all brands of thinners are compatible, as many paints and varnishes brag that their differences are their advantages over one product or another. Generic paint thinner from the local hardware store is almost certainly cheaper, but is not necessarily going to be compatible with your project. Solvents designed for surface prep, dewaxing, or degreasing are also not usually appropriate for adding to paints or varnishes.
Most one-part paints and varnishes will have a dedicated brushing or spraying thinner. Brushing thinners will evaporate slower, giving more working time and a good sense of flow for brushing. The age-old method of adding some mineral spirits is not always a good option, as many specified thinners are actually blends of solvents designed to craft the best working time and cure time. Spray thinners evaporate quickly, and reduce the paint or varnish so it will atomize out the tip of the spray apparatus.
Two-component paints or varnishes are a little trickier, and can have multiple brushing or spraying thinners for different conditions or applications. These paints and varnishes are very sensitive to improper thinning, or use of the wrong thinner.
It is strongly recommended that if you're not sure what thinner to use, look on the label, or contact the manufacturer in advance. More often than not, customers who don't have the right thinner end up very dissatisfied. If you're using a paint or varnish product you've never used before, we recommend that you get the thinner for it as well, as this thinner is also commonly used for cleaning up.