The goal of primer is to create a clean sealed surface that is optimal for accepting a new paint coat. Primers have a greater level of solids and have additives to help with adhesion, all things that make the application of paint go more smoothly. However, the new generation of paints are higher in quality, and some of them are even engineered to be ‘self-priming’, so the necessity of the use of primer is less today then it might have been with older generations of paint. With all that said, there are still times when you definitely want to use a primer:
If the surface has stains on it – this indicates the presence of foreign substances on the surface that can inhibit a paint’s ability to adhere properly
If the surface is absorbent – surfaces like fresh drywall that are absorbent will compromise the finished paint coat if not properly prepared with a primer
If you are drastically changing the color of paint – If your walls were a dark or extremely bright color before (like a dark blue or bright red), and you want to go to a very light color (like white or light grey), then a primer will definitely help in the process. The darker pigments of the original paint will want to show through the lighter pigments, and you often don’t get the desired result. A primer with its thicker base of solids helps to cover up the original dominant pigments and prep the surface for your lighter paint tone.
If the original surface has a high-gloss – A high-gloss surface results in an environment that paint doesn’t adhere to very well. Primer definitely will help in this application, along with a recommended prep-sanding to scuff the gloss surface for better adhesion.