How paint dries
Water based and oil based paints dry in two completely different ways, and that one difference explains most painting advice.
Water based paint is a suspension of tiny plastic particles in water. As the water evaporates, the particles crowd together and fuse into a film. This is called coalescence. It only works above a minimum temperature, which is why cold rooms leave a weak, powdery film that looks fine and then fails.
Oil based paint hardens by reacting with oxygen in the air. The oils link together into a tough network. It carries on hardening for weeks, which is why gloss feels dry in hours but marks easily for days, and why oil goes yellow and brittle with age.
In practice. Keep rooms above 8°C while water based paint dries, and leave oil gloss at least 16 hours before the next coat even if it feels dry.
Pigment volume and sheen
The ratio of pigment to binder decides how shiny a paint is, how well it hides flaws and how hard it wears.
A matt film scatters more light, which reduces reflections and makes unevenness less conspicuous. It does not make defects disappear. Some traditional flat paints burnish easily, but modern durable matt formulations can be washable and scuff resistant.
Gloss forms a more reflective finish, so defects are easier to see in raking light. Durability depends on the binder, formulation, preparation and cure, not sheen alone. Compare the actual product data rather than assuming gloss is always tougher.
In practice. Use flat matt on old, uneven walls and ceilings. Use a durable matt or eggshell where hands and scuffs land. Save gloss for trim that has been properly filled and sanded.
Dew point and surface temperature
Outside, the temperature of the surface matters more than the air. Paint applied to a surface near its dew point traps a film of moisture under the coating.
The dew point is the temperature at which moisture in the air condenses into water. On a clear evening in Fife, a north facing sill can fall below the dew point while the air still feels mild. Paint laid on that sill goes on over invisible dampness.
A 3°C margin above the dew point is a common screening reference, but the product specification takes priority. Measure the surface and monitor conditions during drying. Clearing that margin does not establish that the surface is dry or every other requirement is met.
In practice. Start exterior work once morning dew has gone, stop early enough for the paint to skin before evening, and check surface temperature with our drying timer.
Moisture in timber and plaster
Paint seals moisture in. If the surface is too wet, the water pushes the paint off as it tries to escape.
About 18% timber moisture is a screening reference used in our tools, not approval to paint. Follow the coating maker’s limit, account for meter settings and timber species, and investigate why readings are elevated. A reading cannot predict a fixed date of failure.
Fresh plaster has to dry out before it is painted. A thin skim usually needs one to two weeks in a warm, aired room, and longer in winter. A full replaster on new backing often takes four weeks or more. It should be an even pale colour with no dark patches.
In practice. Never paint timber after rain until it has had a dry spell, and never rush a new plaster wall because the room is needed back.
Film thickness and spread rate
How long a coating lasts depends on how thick it goes on. The spread rate on the tin is the thickness the paint was designed for.
Stretching paint over more area than the tin states leaves a film too thin to protect, particularly outside where ultraviolet light breaks the binder down. Piling it on too thick causes runs, wrinkling and slow, soft drying.
Two thin coats nearly always beat one thick one. Each coat dries properly and fills the gaps the last one missed.
In practice. Work out the litres you need from the area and the spread rate rather than buying by eye. Our paint calculator does this for you.