A quart will do it if you thin it about 50%. I know people who do this, but I have to wonder what that does to the anti-fouling properties. I use two quarts of bottom paint mixed with 1/2 pint of thinner and this comes out just right for me. If you use thinner, don't use the solvent based type that is intended for spraying; this will make it dry too fast when brushing or rolling. Use the type that extends drying time like Interlux brush ease or a product called Penetrol.