Drying Is Not Evaporating: Three Different Mechanisms, Three Different Films

Gum arabic dissolves in water and comes out again when the water goes. Nothing chemical has happened, so adding water re-dissolves it completely. That is why watercolour can be lifted, softened and reworked indefinitely, and why a finished watercolour is so vulnerable — a drop of water on a framed painting damages it in seconds. It also means watercolour has no real drying TIME in the chemical sense. It is as dry as it will ever be once the water has gone, which is minutes. Layers can be built quickly, at the cost of every layer disturbing the one below.

Acrylic paint is solid polymer particles suspended in water. As the water leaves, surface tension forces the particles together and they deform and fuse. Once fused they cannot be separated. So acrylic is workable for minutes and then finished. It cannot be re-wetted, which is why a palette dries into a skin and why a brush left standing is ruined rather than dirty. It also has a MINIMUM FILM FORMATION TEMPERATURE. Below it the particles are too hard to deform and never fuse properly, so the film is a weak powder that looks right and is not. Painting acrylic in a cold studio produces exactly that, and the fault appears later as chalking.

Metal DRIERS — cobalt, manganese, zirconium salts — catalyse the oxidation and can turn a week into a day. They are why 'boiled' linseed oil is called that: it is usually not boiled but dosed with driers. The cost is that a catalysed film keeps oxidising past the point it should stop. Over-catalysed oil paint becomes brittle and yellows more, and it does so over decades — so the fault appears long after the person who added the drier has stopped painting. Use as little as will do, and never add driers to an upper layer if the lower ones have none. That would make the top layer dry FIRST, which is the fat-over-lean violation arriving through a different door.