Checking the viscosity of sprayable paints, particularly car paints, is essential to ensure that your paint goes on smoothly, evenly and easily. The difficulty of checking this is that the viscosity can change depending on the temperature, so the same paint may be thicker at 15.6 degrees C and too thin to use at 80. Another problem is that different brands of paints need to be applied at a different viscosity. This is something that you must find out when you buy the paint (it will usually say on the container or the manufacturers website). Another issue is that some viscosity cups may hold more paint than others so the time may vary. Use the brand recommend on your paint sprayer for the best results.
Acquire a viscosity cup. There are a few brands of these, so look for the one your sprayer is quoting. It will say something like "12-14 seconds Zahn #1" which means you should buy a Zahn #1 viscosity cup. You can find these cups online or from a finish (car paint) supplier.
Dip the cup into the paint so that it completely fills to the top and prepare your stop watch. The pointy end with the hole should be facing downward into the paint.
Pull the cup from the paint, start the stop watch and let the paint run freely. Stop the watch once the cup is empty. The amount of seconds it takes to empty is how you see the viscosity. Run the test again to ensure accuracy.
Compare the amount of seconds it took for the paint to drip through your cup using a viscosity chart. This chart will tell you the ideal time. You may need to either thin or thicken your paint as necessary to reach that time.
Avoid thinning or thickening more paint than you will need within the next few days. Temperature change and chemical interaction can affect your paint and make it difficult to use or even break down in some cases (particularly in extreme heat). You should prepare your paint for immediate use under present conditions and leave the paint you can't use immediately.