"Keying in" is a basic concept that applies to many applications. In general, you are applying a liquid or semi-liquid/wet substance to some solid surface. The point is to achieve great adherence and more consistent, longer lasting results.
It's difficult to show because often keying in occurs at the microscopic or near microscopic level. Three common examples are paint, joint compound, and thinset. Using paint over drywall as an example, what are we trying to adhere to exactly? We're trying to adhere to paper. If you look at paper under a microscope, it looks like this.
You can see all the nooks and crannies. You can imagine how getting the paint down in there would greatly increase adherence and scrubbability. So when applying paint, we're really trying to accomplish 3 things:
1. get the paint from the container to the surface
2. key in the paint to the surface
3. create the desired finish
Sometimes you can accomplish 2 or even 3 of these at once. For example when rolling paint on a wall, filling the roller cover and then rolling the paint on accomplishes all 3, assuming you use a modicum of pressure to push the paint into the surface.
When applying paint with a brush, such as polyurethane to a wood table top, the paint brush and gravity combine to get the paint into the wood (1 & 2). (And polyurethane is thinner than those thick acrylic wall paints.) Then you pull the brush over smoothly in long strokes (the length of the table) to get the most consistent finish (3).
When painting a house with siding, some good painters will apply the paint with a sprayer (the most efficient way to accomplish 1) and then brush it in (the best way to accomplish 2). Sprayers are great at moving paint from the container. They suck at keying in the paint.
When applying an epoxy to a garage floor, the paint can be poured onto the concrete (1). Then a squeegee is used to firmly spread the paint, keying it in to the microscopic craters in the concrete (2). Finally a notched squeegee applies the paint with the correct thickness (3) allowing it to self-level properly (3).
It's difficult to show because often keying in occurs at the microscopic or near microscopic level. Three common examples are paint, joint compound, and thinset. Using paint over drywall as an example, what are we trying to adhere to exactly? We're trying to adhere to paper. If you look at paper under a microscope, it looks like this.
You can see all the nooks and crannies. You can imagine how getting the paint down in there would greatly increase adherence and scrubbability. So when applying paint, we're really trying to accomplish 3 things:
1. get the paint from the container to the surface
2. key in the paint to the surface
3. create the desired finish
Sometimes you can accomplish 2 or even 3 of these at once. For example when rolling paint on a wall, filling the roller cover and then rolling the paint on accomplishes all 3, assuming you use a modicum of pressure to push the paint into the surface.
When applying paint with a brush, such as polyurethane to a wood table top, the paint brush and gravity combine to get the paint into the wood (1 & 2). (And polyurethane is thinner than those thick acrylic wall paints.) Then you pull the brush over smoothly in long strokes (the length of the table) to get the most consistent finish (3).
When painting a house with siding, some good painters will apply the paint with a sprayer (the most efficient way to accomplish 1) and then brush it in (the best way to accomplish 2). Sprayers are great at moving paint from the container. They suck at keying in the paint.
When applying an epoxy to a garage floor, the paint can be poured onto the concrete (1). Then a squeegee is used to firmly spread the paint, keying it in to the microscopic craters in the concrete (2). Finally a notched squeegee applies the paint with the correct thickness (3) allowing it to self-level properly (3).
