Some primer coats will require the mixing of multiple parts while others are incorporated into the top-coat type paint to speed up the process. The greatest advantage to using primer is that it will create continuity on the painting surface that will then help the top-coat layers to appear even and smooth. Especially on porous surfaces, like wood or plaster, a primer is going to prevent uneven absorption of the paint which can distort the coloring and ruin a finishing job. Since not all primers are alike, it is import to consult a professional when selecting the primer coat for any new project to ensure the right combination of surface sealing, protection and color support is going to result.