The first distinction to make is there are actually two types of knife, a palette knife and a painting knife– the palette knife is generally used to mix paint on the palette, and the painting knife is used to apply paint to the paper/canvas.
As you can see from the image, the painting knives come in a variety of shapes and sizes, each one producing a different effect and texture. They can be plastic or wood and metal. I prefer the wood and metal ones myself as they are stronger and therefore make a more consistent mark. Most of the knives have blunted edges, as they are more suited to spreading paint as oppose to using it as a cutting tool.
Apply paint with a knife as you would spread frosting on a cake (delicious!)- thick and textured. You can drag, scrape, and spread the paint in any direction, creating the desired effect suitable to the painting. It is usually best to not thin the paint with any medium beforehand, whether it be oil or acrylic, as you want the paint to be thick so it retains its shape and stays where it is put.
A clean palette knife is important when changing colours. However, it is easy and efficient to clean, as you can simply wipe it on a piece of cloth or paper roll.
For large areas like skies or bodies of water, I recommend using a larger knife so as to cover as much space as possible and spread it around as desired. I usually scoop up a large dollop of paint on the underside of the knife and liberally spread it over the appropriate area. Don’t be tentative with the paint, as one of the joys of this technique is to put it on thick and then move it around the surface.
A medium sized knife can be used on its edge to create effects like reflections, tree trunks, or thin lines to suggest texture and detail.
You can also use the smaller knife tip for subtle details like highlights, distant windows or trees for example. The corners of the blade also work perfectly for fine details.
All it takes is that initial bit of courage to try it and once you’re off, you won’t look back!