Step 1: What you need:
spray paint and/or acrylic paint -- one lighter color and one darker color
newspaper or other coverings
a tiny thin paint brush known as a liner brush
a thick bristled brush
water to clean brushes with
I included pictures of my supplies. As for paint I used spray paint for my lighter base coat. I did this cause my bow was big but acrylic or other paints could be used too. For the darker color, however, you MUST use some form of liquid paint -- NOT SPRAY PAINT. You maybe could come up with a stencil idea for spray paint but it is not going to look as natural as using a liquid paint. I mixed a distress paint and a brown paint together. A distress paint is not necessary but it was something I had around that could darken up the color a bit due to its mossy green color. As for the brushes, the thinner, the better for the small one but the bigger bristled brush can vary in size. I used both the green and yellow handled brushes for big brushes so either size is fine.
Step 2: The model
Step 3: The Process:
Second step: Start painting your v-shaped/line/knot grains on the object using the thin liner brush and the darker coat of paint. I painted my lines while holding the brush at the very end to ensure I wasn't controlling the brush too much. Remember that you are trying to emulate nature and therefore it should not be perfect. Paint a few lines at a time, go to the next step, and repeat. Paint some lines lighter and some lines darker by varying the thickness of the paint to add a more natural look. Make sure to leave half inch or so gaps between lines. After the next step, if spaces look too barren add tiny lines or dots in between.
Third step: After painting just a few lines, use the bigger thick brush to swipe over the still wet lines. Make sure the brush itself is still dry. This will swipe the lines out into smaller lines and it will break up your brush strokes. The longer you wait, the less the paint will be brushed away so try to let some areas dry more before brushing then others to give a more natural effect. Brush the brush in line with the lines if they are very wet to limit how much area the paint spreads to.
Fourth step: Let dry. Try not to let anything else touch your lines so the effect is preserved. For my bow, that meant stringing it up between posts on my deck. At the end you should have something looking realistic and for a lot cheaper than some other methods. Here's a picture that shows what my bow looks like with and without this effect.