Before we start discussing flat paint pros and cons, let us look at what are the basic types of paints used for house painting. Paints are basically divided into two categories, the most used are the latex paints, also known as water-based paints, and the others are oil-based paint.
Water-based paints, or latex paints, or acrylic latex are eco-friendly paints and cheaper, as the equipment doesn't need a solvent for cleaning, but plain water does the job. While the oil-based paint mostly have a hydrocarbon-based solvent that acts as a vehicle, and an alkyd resin. Water-based paints usually dry in an hour or two, while oil-based paints take a long time to dry. Oil-based paints are tough, resistant paints, and ideal for exterior painting. Both, the water and oil-based paint are available in flat finish.
Flat paint is a kind of finish, and has a gloss level which is anywhere between 1 - 9%. First, check if the water or oil-based paint is required for painting, and then decide in what kind of finish you need to buy the paint. Flat-based paint has its own set of advantages and disadvantages. It dries to minimally reflective finish, and is sometimes referred as matte paint too. So, make sure you read the paint can label properly, to ensure it has the right kind of finish.
Flat Paint Vs. Primer
Primer and flat finish are two different kinds of interior paint colors. Primer is a coat of paint which is applied before applying the main paint, which can be water or oil-based. If you are applying an oil-based paint or water-based, then apply a layer of primer paint, so that the next layer of paint adheres properly. Many people tend to use flat finish to replace the primer coat. This is okay, if you have a wall which doesn't have much flaws. Also, this will help you save an extra coat of primer, but if your wall has lots of flaws like cracks, gaps, uneven surfaces, etc., then use both.
Flat Vs. Semi-gloss Finish
Flat finish is good if you have a textured wall, or wall with many flaws, because flat finish tends to hide the flaws due to less reflective quality. But, if you are using flat paint in a high traffic area, then it is a bad idea. Because, areas like kitchen or bathroom need a semi-gloss finish as it is easier to clean and can be scrubbed to a certain extent, and you can prevent the stains. Semi-gloss finish comes quite close to waterproof paint.
Flat Vs. Eggshell Finish
Above was the flat paint and semi-gloss comparison, which tends to confuse a person which is the right kind of paint. If you are confused between both, then consider going with eggshell paint. As it is a good compromise between both the finishes. Eggshell finish gives a little glossy finish, without looking as bright as a semi-gloss finish. Also, it provides better stain protection than a flat finish, and can do okay in high traffic areas of the home.
Now, there is washable flat paint available. However, this product is fairly new. It seems to have many advantages, so you can consider this finish if you want to use a flat finish in bathroom. But, to be on the safer side, you can stick to semi-gloss finish for bathroom.