Materials · Every painted surface · Reading, then a weekend
What sheen actually means
Sheen is simply how much light a dried paint film returns. A matt film scatters light in every direction, so the colour looks soft and the surface looks flat. A gloss film returns light in one direction, so the colour looks harder and every ripple in the wall behind it shows.
That single property drives everything else. Higher sheen generally means a harder, less porous film, which is easier to wipe and harder to touch up invisibly. Lower sheen hides a poor surface and forgives patch repairs, but marks more easily.
Matt and eggshell
Matt belongs on ceilings and on wall surfaces that are not touched: a living room above picture height, a bedroom, anywhere you want the colour to look deep rather than bright. It is the most forgiving finish on old plaster, because it hides the small undulations a flatter light would find.
Eggshell is the compromise most houses end up living in. It has just enough sheen to take a wipe and just little enough to stay soft, which makes it the sensible choice for a hallway, a child's room or any wall that will meet hands. It is also easier to cut in neatly than a true matt, because the wet edge stays open a little longer.
Satin and gloss
Satin and gloss belong on woodwork: skirtings, door frames, doors, window boards, and anywhere that will be cleaned rather than repainted. The harder film is the point. A satin skirting can be washed several times before it needs attention; a matt one cannot be washed at all without polishing a patch into it.
Gloss shows every defect underneath, so the preparation matters more than the painting. Fill, sand back, dust off, and prime bare patches before the top coats. If the substrate is poor and you do not want to spend the afternoon preparing it, drop to satin and the surface will look considerably better.
Testing and recoating
Compare sheens on primed card rather than on the wall, and look at them at an angle rather than straight on. Sheen is almost invisible face-on and obvious in raking light, which is exactly the light a hallway wall gets in the evening.
Respect the recoat time printed on the tin. Rolling a second coat onto a film that has not properly hardened will drag the first coat and leave a patchy sheen that no amount of further coats will even out.
Keep a labelled record of what went where. The sheen is as important as the colour when you come back to touch up a wall two years later, and it is the part nobody ever remembers.