Paint does not hide anything, it points at it
The instinct is that a coat of paint will even things out. It does the opposite: fresh paint gives a wall a uniform colour and sheen, and a uniform surface is exactly what makes every dent, ridge and filler mark visible, because there is nothing else for the eye to look at.
So the finish is decided at the preparation stage, and roughly in this order: fill and sand the dents, caulk the gaps where walls meet trim, wash anything greasy, and sand back any glossy surface enough to give the new coat something to key to.
The sheen decides how much of this you have to do. Matt scatters light and forgives a lot. Every step up in sheen reflects more, and reflection is what reveals a defect. Choosing eggshell or satin is choosing to do more filling, and that decision comes before the filling rather than after it.
Ceiling, walls, woodwork
Rollering a ceiling throws a fine mist of paint downwards over everything in the room, so the ceiling goes first regardless of anything else. Walls next. Woodwork last, cut neatly against a wall that is finished and dry.
Some decorators reverse the last two, painting trim first and then cutting the wall colour down to it, which is a legitimate preference and slightly faster if you are quick with a brush. The part that is not a preference is the ceiling.
Cut in one wall, then roll that wall
This is the single most useful habit on the page, and getting it wrong produces a defect that people live with for years without ever working out what it is.
A brush and a roller leave different surface textures. While both are wet they merge into each other and you see nothing. Once the brushed band has skinned over, the roller cannot blend into it, and you are left with a border of one texture round a field of another.
The fix costs nothing: cut in one wall, roll that wall straight away, then move to the next. What causes the problem is the tidy-looking approach of cutting in the entire room first and getting the roller out afterwards.
Keep a wet edge, and stop going back over it
Work in manageable sections and always roll back into the area you have just done while it is still wet. Emulsion begins grabbing as it dries, so going back over a half-dry patch to fix a thin spot tears the surface and leaves a mark worse than the one you were fixing.
If you can see a thin area and the paint has started to set, leave it. The second coat covers it. Almost every roller mark and lap line in a finished room was made by somebody trying to improve something at the wrong moment.
Two thin coats beat one thick one, every time. A heavy coat runs, dries unevenly and takes far longer to harden.
Two coats, and what that does to the quantity
The number on the tin is coverage for one coat on a prepared surface, which is why so many people end up short. Two coats over a colour change is the normal case rather than the cautious one, and a strong colour going over a pale one, or the reverse, can want three.
The paint calculator works that out, including the two things that make most paint estimates wrong and the separate case of freshly plastered walls, which need handling differently before any topcoat goes near them.
If you are still choosing, the room visualiser puts a colour on a wall, with the honest caveat that a screen emits light and a wall reflects it, so no visualiser can promise you the colour. And room area gives the measurements everything else starts from.
Common questions
What order should I paint a room in?
Ceiling first, then walls, then woodwork. The logic is gravity and overspray: rollering a ceiling throws a fine mist downwards onto everything below it, so anything already finished gets speckled. Woodwork last means you cut the gloss or eggshell neatly against a wall that is already done and dry. Some decorators do trim before walls so they can cut the wall colour to it instead, which works too. What does not work is doing the ceiling after anything else.
How long should I leave between coats?
Whatever the tin says, and the tin is a minimum rather than a suggestion. Emulsion is usually touch dry within an hour and ready to recoat in two to four, but recoating while the first is still soft drags it and lifts patches. The other half of the answer is that a second coat applied too soon over a slow-drying surface stays soft underneath and marks easily for weeks. If the room is cold or humid, everything takes considerably longer than the tin assumes.
Is it better to use tape or cut in freehand?
Freehand is faster once you can do it and tape is more reliable if you cannot, so the honest answer depends on how many rooms you paint. What tape does badly is edges that are not sharp: on textured or uneven surfaces paint creeps underneath and the line is worse than a decent brushed one. If you do tape, press the edge down firmly and pull it while the paint is still slightly soft, because peeling fully dry paint takes flakes off with it.
Why can I see roller marks or a band round the edge of my wall?
Almost always because the brushed edge dried before the roller reached it. Brush and roller leave different textures, and while both are wet they blend. Once the brushed band has skinned over, rolling up to it leaves a visible frame around the wall, which shows up in raking light from a window. The fix is order rather than product: cut in one wall, roll that wall immediately, and only then move on. Doing all the cutting in first and all the rolling afterwards is what causes it.
Do I need to wash the walls first?
In a kitchen absolutely, and in most other rooms it depends on what is on them. Cooking leaves a fine grease film that paint will not key to, so a kitchen wall painted without washing can look fine and then peel in sheets months later. Sugar soap and a rinse is the traditional answer. Elsewhere, dust and cobwebs matter less but nicotine, mould and anything greasy around light switches and door handles all need dealing with rather than covering.
Does the finish change how much preparation I need?
Considerably, and it is worth deciding the sheen before deciding how much filling to do. A matt finish scatters light and hides a surprising amount of unevenness. The shinier the paint, the more it reflects, and reflection is what makes a dent or a sanding mark visible. So a wall you would be happy with in matt can look poor in eggshell or satin, and choosing a higher sheen commits you to better preparation rather than the other way round.