Is sunscreen effective after face foundation is applied on top?

Sunscreen is most effective when applied directly to clean, dry skin before any makeup, including foundation. If you apply foundation on top of sunscreen, it can reduce the sunscreen's effectiveness, particularly if the foundation is heavy or if it's applied in a way that disturbs the sunscreen layer. For best protection, consider using a makeup product that includes SPF or reapply sunscreen over makeup with a setting spray or powder sunscreen designed for that purpose.