The primer before the paint application seals the surface so that the paint cannot penetrate. The primer forms the perfect base for the paint to be applied. The primer also provides the ideal surface for the paint to adhere to. On a stained piece of furniture, like my china cabinet, the original surface is usually shiny so the paint wouldn't stick to it. Applying a primer to a shiny surface is the ideal base for a beautiful new color. The same technique can be used on tiles, glass, vinyl siding and aluminum - all smooth surfaces that would not be painted without a primer!