A question I am asked regularly is, “Why can`t I wash the dirty marks my children have made off the walls and when I try to, the paint comes off where the marks are?” Well the answer is simple, you probably have the wrong type of finish on the walls.
If you have matt emulsion on your walls, especially if it is a contract emulsion (used on newly built properties, not a vinyl based emulsion) you are going to find it hard to wash them over. Matt emulsions dry to a rough finish, not smooth like the more shiny paints. This makes them hard to wash as the dirt gets trapped in the microscopic pores in the paint surface. That is why undercoats are flat matt, as they make a good surface for the topcoat above by allowing the topcoat paint to sink into the pitted rough surface of the undercoat. Contract emulsions are even worse as they do not have much vinyl in them (i.e. plastic) used to bind the paint pigments together and so are chalky so wash off easily when you try to clean the dirt.
If you want a more washable surface go for the more shiny emulsions (not everyone’s cup of tea) like Silk or mid sheen, Dulux call their version Soft sheen. These have a smooth surface, not pitted and so reflect light making them shine as well as smooth and washable. Silks are very shiny when first applied but do tone down later on but are excellent for durability and wiping over as the more sheen the more washability. A lot of people do not like the high sheen as it will highlight wall defects. A good compromise are the mid sheen finishes, not as shiny as silk (it has a softer sheen) so does not show up the defects so much but is still washable. As shiny paints are also flexible they cope well with both slight surface movement and do not split like matt paint can do over flexible mastics fillers.
So there you have it, matt emulsions have one real advantage, they give a non-reflective surface that does not show any surface defects and always looks good when dry no matter how poorly applied. Shiny finishes are more durable, washable and flexible but do show up surface defects, which some people do not like. Remember matt emulsions are not recommended for use in areas of high steam or condensation like kitchens and bathrooms where as silks and mid sheen are.
Just a quick word about the diamond glaze finishes or washable matt paints (10x tougher than normal emulsion paints) that are available and becoming popular, they are not really emulsion paints but acrylic eggshells so I am not going to cover them much in this section. I will say is they are very hardwearing and tough but difficult to apply and not as easy to use like emulsion paints and well over the top for normal interior walls.