Pilling gets worse when I layer foundation over sunscreen — whose problem is it?

Usually the interface, not either product on its own.

A mineral sunscreen built on a W/O emulsion leaves a film that a water-based foundation then sits on. The two films shear against each other as they are rubbed, and the top one rolls up. The same sunscreen alone can be perfectly fine.

Three contributors. Film-former incompatibility across the two products is the common one. Too high a solids load with too little elastomer or oil to bind it leaves loose powder at the surface, which pills readily. And over-rubbing during application sets the film and then breaks it up.

Fix from both sides: reduce solids or add a film former on the sunscreen side, add a small amount of spherical filler or boron nitride for slip so less rubbing is needed, and on the foundation side avoid a film former that is incompatible with the one underneath.

Test the way customers use it — full skincare routine, then sunscreen, then base, on the same panel at a fixed dose. A single-product test will not reproduce it, which is why this complaint arrives from the field rather than the lab.