Finishes vary in viscosity (thickness in liquid form) depending on their type and formulation, and especially in temperature differences. The viscosity is thicker when the finish is cold and thinner when the finish is warm.
To measure viscosity dip a viscosity cup into a finish so the cup is full. Then lift the cup out of the finish and begin timing the drainage with a stopwatch. When the stream breaks, indicating that the cup is empty, stop the timer. The number of seconds is the measure of the viscosity.
There are many types of viscosity cups. Some are quite expensive. The most common cups used in finishing are the Ford #4 and the Zahn #2. You can go online and find conversion tables.
Finish manufacturers sometimes tell you the viscosity at which their finish should be sprayed for best results. But I’ve never found these numbers to be more than just general guidelines because the type of gun, air pressure and whether you’re using an air compressor or turbine, all make a difference.
I’ve found viscosity cups more helpful for dealing with temperature differences. If you determine that you get good results (usually meaning minimal orange peel) at a certain temperature and number of seconds drainage through a viscosity cup, it’s easy to check against this when the temperature is different. You’ll know if you need to add thinner or increase or decrease air pressure.