My better half says "JUST PAINT". It seems so easy, just squeeze some paint onto a palette, grab some brushes (and maybe some medium), throw a canvas up on the easel and start spreading that stuff around! BUT. If a painter doesn't have an understanding of the materials, the results can be something like a backyard mechanic pulling the pieces off an engine without knowing how they go back together. There are basic mechanics involved in any art form, and the more the artist understands the materials, the sounder the process in general. For painters, specifically OIL painters, it's necessary to know a number of rules relating to the support (what the painting is painted on; wood, canvas, hardboard) and how it's prepped (sealer or sizing, oil or acrylic ground, how many layers, what kind if any texture); whether to use an underpainting and how complex it should be if so; how much and what type of medium to add and it's composition; which paints are transparent, which are opaque, which dry faster, which are very slow; what kind of brush to use for a particular paint application, how thick or thin the paint goes on, what kind of brushstroke.. you get the idea? Now let's go deeper, into all the different pigments and types and their characteristics, how they blend and interact, which are archival and which are suspect, and who makes the best paint, which is always good for a rousing discussion!
It's like the world of coffee home roasters, where type of bean, specific varietal, where it's grown, how it's processed, how it cupped, ect. is just the beginning before we roast. Then we have degree of roast (first crack, second crack, and how long into it), what type of roaster, how to ramp up and control the roast cycle, etc. to consider. I have enough experience to roast by the seat of my pants, and it's always good if I keep a careful eye on it. (As in never walk away from a roast in progress, lest you hear the dreaded third crack, which is the glass of the roasting chamber!) But we don't want to go there, I can bore anyone who isn't also a home roaster to beyond tears, and have.
I paint by the seat of my pants now and then too; some paintings beg for bravura brushwork and experimentation, but it's better to have that basic experience and knowledge from which to pull when doing so, or one can end up scraping off lots of wet paint! (Though sometimes that can make for a lovely resurrection of the work).
So good painting requires study and knowledge of a tremendous number of art related areas. One method of direct study is to do paint tests and comparisons, one of which I have posted here to share some of the behind the scenes process. It may look like I'm not painting when there are no new canvases up, but it often means I'm busy doing some homework!
This was a study and comparison of Vasari, Michael Harding, Puro (and a 30 year old tube of Brera, by Maimeri, who makes the Puro) with some 25 year old Grumbacher Pretested earths I have. The top of the color swatch is straight from the tube, the middle is thinned with turps to check pigment load and luminosity, the bottom is mixed with my standard medium. (I am also doing a drying test with this panel). They all have their own strengths, and this panel is now part of my reference tools for what colors I want to favor, and for how I want to use them.