There are two different ways of spray polyurethane. One is to spray on a vertical surface. The other way is to spray on a horizontal surface. The tip size we should use is different in both cases. 1. For a vertical surface, we should use a tip size between .012 and .016 inches. This is the size we use when we paint the ceiling. 2. For a horizontal surface, the tip size we should use is between .021 and .029 inches. This is the size we use when we paint the wall. We are supposed to consider the thickness of the material we are spraying. We should use a tip size that is appropriate for the thickness or material. The .012 and .016 tips are used for thickness up to 1/16. The .021 and .029 tips are used for thickness up to 1/8. If we want to use a tip size that is between these numbers, we can use the rule of 3 to get the exact size. For example, we want to use the tip size to spray the material that is 1/16 thick. We can use the 1/8 +1/16 rule to find the exact tip size. The size we are looking for is .049 inches.