One benefit of working with latex paint is its ease of use. As soon as you bring the paint back from the store in mixed condition, latex paint is ready to go.
New, freshly mixed latex paint rarely needs to be thinned for most paint roller and brush applications. Paint manufacturers produce latex paint of a viscosity that works for an average number of applications. Since “average” does not apply to every situation, you may need to thin latex paint for applications such as paint sprayers or for older paint that has partially lost water content due to evaporation.
How Does Water Thin Latex Paint?
Paint is divided into two categories: oil-based (or alkyds) or water-based paints. Oil-based paints should be thinned or cleaned up only with petroleum or mineral-based products. In contrast, latex paint is water-based and should be cleaned up and thinned only with water.
Water is the carrier of solid products in latex paint. Successful latex paint thinning requires a reasonably precise assessment of paint viscosity and the addition of the correct quantity of water.
Source: The Spruce