Try not to apply paint in extremes of temperature but if you do, you must except that you may not get a perfect finish, no matter how good the paint. Too cold or humid, it will take an age to dry, and if allowed to become wet or freeze, it
will ruin the finish.
Too hot, and you will quickly loose your wet edge, the paint surface will dry very quickly (possibly, in the time it takes to reload your roller or brush) and will look patchy. This can be a particular problem with modern high opacity emulsion paints, especially when painting a ceiling in the middle of a summer heat wave but it will also affect solvent and water-based trim (used on wood and metal) paints. In the case of a water-based paint, about 5% of water can be added to help extend the wet edge in such conditions.
Paint, when applied onto a nonporous surface, dries from the outside, in. The first drying stage is ‘Touch Dry’; this is when the paint appears not sticky to the lightest touch but still soft underneath. Paint will continue to harden as the carrier medium (this is either the solvent or water, which holds the paint in suspension) evaporates into the atmosphere. When the paint appears to be fully dry to the touch, this is known as the ‘Dry Time’ and the point at which the paint becomes ‘Recoatable’ (i.e. time quoted on the tin). This is the earliest point when a further coat can be applied without damaging the previous one. Whilst the paint appears to be dry, it is not hard and will continue to harden over a period of time. This is why we ask you not to wipe clean paint for at least 7 days after application. Some solvent based paints, depending on applied thickness, can take over a month to become fully dry, so don’t rub too hard.