اللمعان والنعومة

Usually build-up or hard water, not damage. Try a clarifying wash before buying a treatment.

Both repair inside the strand with different chemistry. Olaplex N3 is a pre-shampoo treatment you rinse; K18 is a leave-in you don't. If you colour or bleach regularly, either is worth it. If your hair is simply dry, neither will feel worth the price against a good mask.

It makes it look shiny immediately by smoothing the surface, and it changes nothing underneath. Useful as a finish, not as a repair.

Every one to two weeks at most. Protein on hair that doesn't need it makes it stiff and dull — which people then treat with more protein.