Usually when we paint, we concentrate first on the largest shapes, and don't get to the details until the end of the painting process. In workshops that I teach, I see many students struggling to finish a painting well, because of the lines needed. For a sailboat, for example, the rigging must be painted with a light touch to be convincing. But I often see thick, wavy lines painted on an otherwise excellent artwork that ruin the overall result.
So, what's so hard about painting lines? The answer is making them look "painterly" - artistic, loose-looking, and not awkward. Here are some approaches I've learned to paint good-looking lines.
Lines are easiest to paint with a proper brush, and there are three that I use in various circumstances - a rigger (thin, long hairs ending in a point), a script (thin, long hairs trimmed flat at the end), and a "bulb rigger" or "needle-point" brush (like a rigger, but with a bulge of extra hair near the ferrule to hold more paint). A normal round brush with a very sharp point can also be used.
I find a rigger or needle-point brush best when painting
thin lines. My advice is to do all lines, both straight and curved, freehand. This gives the best line quality. (If you
must use a ruler, hold it at 45 degrees to your paper or canvas, and run the brush ferrule along the edge of the ruler.) Load your brush with paint and try a few practice lines on scrap first.
Use a light touch and go quickly - lines look most painterly when they're slightly broken, so don't press too hard. And the most important thing is: do not look at the point of your brush,
look at its destination. This is not easy to do at first, but it's important. If you look at the brush point as you're painting your line, I guarantee your line will wiggle in an ugly way. It's not a big problem if your line doesn't go exactly where you wanted it to. Slightly missing your exact goal will make your painting look looser and more painterly. It's usually better to leave a poor alone than to try to correct it.
Another suggestion I have is to
reposition your paper or canvas to take advantage of the natural swing of your arm. If you're right-handed, it's easiest to paint a smooth line diagonally from your lower left to upper right as shown in the photo above, so move your painting so that the line goes in that general direction. I find it difficult to paint smooth vertical thin lines without turning my paper at an angle. Horizontal lines are easier to do without moving your painting.
I use different methods to paint
thick lines. These look best when broken or "skipped", especially for objects like wooden masts. One approach is to use a rigger or script brush, load it with paint, and lay it on its side along the line to be painted. It can then either be dragged along the line, or "patted" along the line, to make the line interesting. I've also used the edge of a credit card or razor blade dipped in paint and moved slightly sideways along the line to give an interesting texture.
Finishing a painting should be the most enjoyable part of the painting process, and you don't want to spoil an otherwise good work with ponderous, wiggly lines when thin, painterly lines are called for. So
practice painting lines before you add them to your almost-finished painting!
