Should I use a 1.3 or 1.4 tip for base coat?

Whether you should use 1.3 or 1.4 tip for base coat depends on paint type, viscosity, gun type, and desired atomization, but both 1.3 and 1.4 mm tips are commonly used for basecoat and both work well, with slight differences. A 1.3 mm tip is slightly smaller and provides finer atomization, less paint flow, and more control, making it ideal for metallic basecoats, waterborne basecoats, and when you want thin, even coats with less risk of runs and mottling, especially with HVLP guns, and is preferred by many pros for basecoat because it gives smoother finish. A 1.4 mm tip provides slightly more paint flow, slightly faster coverage, and better for slightly thicker solvent basecoats or when you want to cover faster, and is more forgiving for beginners, but can increase risk of runs if you move too slowly. Many paint manufacturers recommend 1.3 to 1.4 mm for basecoat, with 1.3 for waterborne and 1.4 for solvent. If your basecoat is thin and well-reduced, 1.3 is better. If it is thicker or you have larger panels, 1.4 covers faster. Most painters own both and test on cardboard, starting with 1.3 for basecoat for finer finish, then switching to 1.4 if coverage is too slow. So for best finish, use 1.3 for basecoat, especially metallic.