Primer paint is a base coat that is put on walls and woodwork to cover a darker color when going light, and to give the new paint a surface to "bite" into. It should be put on gloss paint if switching to flat. When painting with primer, you don't necessarily get an even coat, and it may go on thicker in some areas than in others. This doesn't have a negative affect on the final paint job. Flat paint is a finish paint, that is usually applied in a 1 coat coverage, and gives a nice, neat, even appearance.