How would you thin down water based paint for spray gun?

I used to work with my dad, he was a commercial and residential painter. He had a spray gun he used. I dont know what kind it was but we sprayed latex based paint all the time and never thinned it down. I would try spraying without thinning. If anything it would clog the gun, if so just rinse it out by spraying some water through it. You could use water to thin the paint but its not going to cover as well. If you do that you need to mix it very well. I hope this is helpful.