(Artist tape is preferable over "regular" tape because it is easily removable and won't leave behind any visible residue.) I apply the tape gently but firmly where I want to paint a straight line, and then apply the paint right next to it, with part of the paintbrush (and the paint) going over the edge of the tape.
Here's the real trick for perfectly crisp lines: seal the edges of the painters tape with some of the base wall color before painting on the contrasting color. Don't paint the whole width of the tape. Just a bit along the edge of the tape on the side where the stripe will be painted.
Avoid using a fluffy roller. These will increase paint bleed significantly. If the stencil doesn't appear flat on your surface, the dense roller will press it down to create a crisp line. Just like with spray paint, spray adhesive can help secure edges of a stencil cutout to the surface when using wall paint.
Answer: For thin lines, you'll likely need a #8 brush, or smaller.
Dip the tip of your toothpick in the paint and drag the tip into the surface that you're painting to apply an extremely thin line. You can use the sharp point of the toothpick for barely-visible lines, or the flat edge on the side of the point for slightly larger lines.
Flat. Flat brushes are versatile. Their long bristles can lay smooth patches of color; make long, bold strokes; or, when using their edges, execute fine crisp lines. Marks made with a flat brush have a distinctive square edge.
The only edger tool you need to paint the edges of your walls and ceilings is an angled paintbrush. When you use this brush, you will find it much easier to get neat cutting in. That's because the angle of these brushes gives you more control and precision in tight corners.
The secret to crisp, clean lines: paint over the edge of your masking tape with the existing color of the wall (or whatever surface you're painting). By brushing over the edge of the tape, you're effectively sealing the paint, and any paint that bleeds through will match the existing color.
For stenciling, a sponge is often considered better than a brush because it allows for more even coverage and reduces the risk of paint bleeding under the stencil. Sponges can also create a softer texture and are generally easier to control, making them ideal for achieving clean, crisp edges.
The primary technique for painting a straight line is using painter's tape.
Named after their original use in painting ship riggings, rigger brushes have long, thin bristles that are perfect for drawing fine, continuous lines. They are excellent for adding details like hair strands, grass blades, or wires, where length and flow are necessary.
Walls, Wood, and Metal - Small 1/4″ roller covers or foam rollers will produce the smoothest finish. Light to Medium Textured Surfaces - Microfiber rollers are best.
Use mediums: Most situations you can just thin your paint with water, but if you find your paints dry too fast, it can risk brush strokes. Flow aids or retarder medium can increase the drying time, and allow smoother applications. Use the right brush: the texture of the brush may also leave brush strokes.