Eyelash primers are applied before mascara. Think of them as structural enhancements. They actually add fibers to your individual lashes, lengthening and thickening them. Primer also makes a smooth undercoat that holds mascara better, so you'll typically use less and still get better coverage. Some mascara products include primerlike ingredients in their formulations, but you might prefer the cleaner look of a two-step process: lengthener (primer) and then mascara. Primers are sold individually, but you may find them as part of a two-product application kit.