Our paint label suggests we reduce the mixture 10 to 15 percent prior to spraying. Reducer is just a fancy name for paint thinner. It makes the paint flow more freely through a spray gun. Some paints do need thinning, too. Paint that's too thick will come out in globs and won't level out on the surface. If we were real geeks, we'd have figured out the volume in our paint cup and taken 10 percent of that. We then would have added that amount of reducer to the mix. Honestly, though, we just guesstimated it. You can tell after a while what the best viscosity is by watching how the paint runs off the stir stick. Paint should be slightly thicker than the consistency of water.