Paint should always be of the thin consistency to be enough to provide a smooth and uniform jet. Many best sprayer brands provide viscosity cups with the product, which helps thin the paint suitable for the particular gadget.    Many HVLP or airless sprayers use VacuValve technology, which accepts paints of any thickness to thin them down internally. But overall, it is better if you use the proper consistency to avoid spraying large droplets. You can just dilute with normal water or use special paint thinners like mineral spirits, turpentine oil, or acetone.