Q: Why do waterproof zippers feel stiff to pull?
Stiffness is built in. The coating or film increases friction between the tape faces, the slider is machined to tight tolerances to limit leakage, and the chain profile is often designed with a deeper interlock than a standard coil. Add a sealed body or a gasket and the effort rises further. A new waterproof zipper can feel two to three times heavier to pull than the same size of ordinary coil.
The sensation usually improves with use as the surfaces burnish, but it should never feel notchy or require two hands. If it does, check that the slider is the correct size and hand, that the tape is not folded or trapped at the seam, and that the chain is clean. Detergent residue and sand are common causes of a sudden increase in drag.
For product design, place the puller where the wearer can grip it, add a cord or tab, and avoid sharp curves at the top of the opening. A reverse-coil chain under a storm flap can deliver most of the weather protection with noticeably lighter action, which is often the better compromise for everyday outerwear.