Especially when spraying, the viscosity of the finish is important and must be matched to the needle size (that is, the opening at the nozzle.)   Each finish or gun will have a recommended viscosity that you may need to obtain by thinning (with the appropriate thinner, of course).  You dip the cup in the finish to  fill, lift it out and then count the number of seconds it takes to drain.

 

 

Look for specifications like this (for a Zahn cup)

Viscosity (Zahn #2 at 68°F):  24 ± 3 seconds

 

 

Thanks!

So that is what those are for, good to know, I thought one hole size did everything. I wonder if the hole sizes on spray cans are sver checked, some paint doesn't want to come out.

Herb