Most of us do-it-yourselfers attempt to paint projects because painting is easy and gives a big bang for the buck. A fresh coat of paint can change the appearance of a surface and make everything look new.
Since paint is so great, why would you ever need to use a primer? Why waste time and money?
Well, if the surface has stains or other problems, using primer is never a waste. It can prevent disasters. A stain could quickly push through a fresh coat of paint and ruin the new surface.
Always prime when the existing surface has stains, chalking, dark colors or irregular surface conditions.
Primers work because they are specially formulated with additional binders, additives, and tougher pigments that cover problem areas.