Foundations and concealers are essential cosmetics when it comes to a flawless make up.
This question not only relates to your skin care routine, but also your makeup. Before you even begin fine-tuning your makeup job with contouring, highlighting, blush and eye works, you have to lay your foundation (literally)!
A foundation is a skin colored makeup applied to the face to create an even uniform color to the complexion, to cover flaws and sometimes to change the natural skintone while a concealer is meant to be used on small discolored areas (Underneath the eyes, around the nose, on blemishes such as red marks from acne).
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Foundation and concealer work hand-in-hand. However, it is best to apply foundation first then concealer because with the foundation applied first there is little need for concealer.
The only exception is when using treatment concealer to cover blemishes, or scarring. In that case you will apply the color corrector or treatment first, but if the concealer is used first it is likely that most of it would be gone while applying foundation, you will end up using a lot more than necessary and can risk looking cakey. No one wants that!