PAINT THINNING RECOMMENDATION Thinning ratio for paint & primer is 4 parts paint or primer & 1 part thinner. Most of our customers prefer XYLENE. If you have a Gallon of paint or primer that has set for some time, you will need to scrape the bottom of the can and put it on an agitator for a minimum of 10 minutes to ensure the proper quality. XYLENE can be purchased at your local hardware store.