If you ever lie awake at night worrying that the paint on the ceiling might fall on you – this is for you.
Why does the paint stick to the wall in the first place?
One place I never had any trouble sleeping was in chemistry lessons at school, but after a few decades working with paint I am more interested these days.
There are actually two different effects. First, wet paint stays up for the same reason that you get wet when it rains. This adhesion is caused, simply, by the molecules in the wall and the molecules in the paint deciding that they like to be close. Sweet.
Adhesion is very general in liquids, while molecules that prevent sticking are quite rare – like the molecules in a non-stick frying pan.
Once the paint is exposed to the air its liquid component (the solvent, now usually water) evaporates, leaving the pigment and the binder (usually some form of resin). The binder, once out of its liquid solvent, undergoes a molecular change, making it bind together and become stronger.
At a molecular level the walls are porous, and the liquid paint will actually partly go into the wall. Very hard surfaces are harder to bind to because the paint attaches very strongly to itself, but much less strongly to the covered surface.
Plaster walls can develop a chalkiness on their surface, and the paint won’t stick well because the chalk is loose. The usual solution, if the problem is not too bad, is to wash the walls down well and then apply a good latex primer that will adhere easily.
When dry, there is a degree of molecular connection between the paint and the wall. The better the paint, the better the resins and the better the connection.
All of this knowledge may seem to have little practical use to your average decorator, but it helps to have an idea what’s happening when you transfer the paint from the can to the wall. The point is to make it stay on the wall after you’ve finished.
Men in white lab coats devote their lives to making all this work, after all.
In the meantime, if you want some new molecules stuck to the inside or outside of your house, we are your men.