Like all cosmetics, foundation primer has an expiration date. As a general rule, it’s best to toss foundation primer, as well as foundation, that’s been open for more than a year. By that point, bacteria or mold spores may well have taken hold in the product, and you might also see a breakdown in the primer’s consistency. Store your primer in a cool, clean, dry spot to keep it from separating or spoiling sooner than you’d like.