Hello Jz,
Using the right tip size is important to getting a great finish. The first digit in the three digit tip size, in this case it would be the first 3 in 313, is half the width of the spray fan when the tip is a foot away from the surface. A 313 tip has a 6 inch spray fan. The final two digits, 13, are the size of the hole in the tip in thousandth of an inch. A 313 tip has a 0.013 inch hole. A 515 tip has a 10 inch spray fan and a 0.015 inch hole.
Thanks for sharing your experience.
-KR