There are many factors affecting the drying time of interior paint. The humidity and temperature when applied has a significant effect. The method of application makes a difference, as does the type, sheen and even colour of the paint. Latex paint dries faster than oil-based paint, but there is a difference between "drying" and "curing." Most people don't realise that latex paint takes significantly longer to cure than oil-based paint.
The two types of paint used for interiors are oil-based paint and water-based (latex) paint. Latex is by far the most common choice for interior painting, but oil-based finishes are sometimes used for wood trim, metal architectural features like wrought iron railings and as primers. Both types are available in a range of sheens, from flat to high gloss.
No matter which type of paint used, the biggest determinant of drying time is the air temperature and humidity. The more moisture there is in the air, the slower the paint will dry, as the liquid in the paint will evaporate more slowly. A room painted with water-based paint in arid Arizona will dry at least twice as fast as the same room painted on a rainy day in Oregon. Water-based paint must be applied when the temperature is at least 10 degrees Carenheit. Cooler temperatures will retard drying time. Oil-based paint can be applied at or above freezing, but will dry exceedingly slowly. A breeze or the use of a fan will speed up drying time, because it increases the evaporation rate of the liquids.
Under average drying conditions, water-based interior paint will dry to the touch in about 1 hour. Oil-based paint will take 3 to 4 hours to dry. When doing more than one coat of paint, leave water-based paint to dry for 3 to 4 hours before applying the second coat. Oil-based paint generally needs about 8 hours before recoating.
Several other factors influence the drying time of interior paint. The flatter the sheen, the slower the paint will dry. The higher solids in shinier paint speed drying time. Some deep colours will dry more slowly because of the amount of tint used. Paint will dry more quickly on porous surfaces, like drywall, because the substrate absorbs some of the liquid from the paint. Paint applied in a thicker film (more common when spraying) will dry more slowly than paint applied with a brush.
Even when dry to the touch, paint has not fully cured to its final hardness. Oil-based paint will cure hard in 3 to 4 days under normal conditions. However, it can take latex paint up to a month to cure hard. Even though flatter paints take longer to dry to the touch, they cure more quickly than glossier paint. Typically, higher quality and more expensive paint cures to a harder and more durable finish than cheaper paint.
Avoid using regular masking tape on freshly painted surfaces, as it's likely to pull the paint from the wall when removed. Instead, use "low tack" blue painter's tape. Don't wash painted latex surfaces until they have cured for at least two weeks; better yet, allow a full month. Wiping or rubbing uncured surfaces will create smudges and dull spots.