Why does a felt-tip liner usually make a cleaner wing than a brush?
It comes down to how each tool releases product. A felt tip holds ink in the fibers and lets it out at a fairly constant rate under light, even pressure, so your line width is set mostly by the tip shape and the angle you hold it at. A brush is the opposite. The amount of product and the length of bristle touching skin both shift as you move, so a clean line means controlling those in real time. That's the harder skill, which is why a brush can do things a felt tip can't once you're good with it, but a felt tip gets you to a clean wing faster. The part most people miss is the surface, not the tool. Liner drags and skips over anything emollient, so a creamy concealer or dewy primer on the lid will make even a good pen look ragged. A light dusting of powder on the lid first gives the tip something to grip, and that's usually the difference between a crisp line and a patchy one. The other thing worth knowing is that most wings fail at the same step, opening the eye before the liner has set. Waterproof and longwear formulas need a few seconds to dry down since they're built to stop moving once they do. Look down into a mirror and hold still until it's dry instead of blinking it into the crease.