Pour the paint into a larger bucket if you are planning to thin out the entire gallon of paint. Allow a few seconds for the extra paint to drain out of the can.
Pour in 1 cup of room-temperature water to the bucket and stir the paint with a paint stick. Stir for several minutes until the mixture is smooth and uniform. Check to see if the thickness is suitable for the application you are using it for.
Add more water, 1 cup at a time, stirring well after each addition. Be careful not to over-thin the paint, or it will not provide proper coverage and might actually flake off later.
Check the paint by applying it on the surface you want to paint, using the tool suitable for the job, whether it is a sprayer, a roller, a paintbrush or a rag. Keep the paint as thick as possible for the best results.