I often use a palette knife to paint lines, sunlight on a tree trunk, the sunlight on a cactus, branches, electrical lines, any kind of highlight. I seem to find it easier to get a fine, straight line with a palette knife rather than the brush. It seems whenever I pick up the brush for detail, the stroke is heavier than I want.
Mix up some oil paint, put a small amount of medium in the paint so that it isn’t too stiff. Then load up the side of the palette knife with an even amount of paint. Now the trick is to get the paint where you want it and not beyond. I found that if I place a razor blade or another palette knife down where I want the stroke to end, the blade or knife acts as a sheild or mask keeping my line of paint just where I want it.
Another tool that I use is a rubber tipped sculpting tool or a brush that is sharpened like a pencil on the end. You don’t want the point too sharp or you won’t see the line. I usually use the sculpting tool. I can use this for branches, grass texture or lines. It is especially effective in dark paint, scraping away paint to get down to the light canvas. I even use it to sign my name in dark passages.
The technique of scratching out a layer of paint to reveal the color underneath is called sgraffito.