Achieving a crisp, professional finish when painting internal and external corners—known as inside (concave) and outside (convex) corners—requires specialized brush techniques. These angles are high-visibility areas where imperfections like drips, uneven coverage, or brush marks are most apparent. Unlike painting a flat wall, corners demand a methodical approach to manage paint application, maintain wet edges, and create sharp lines without the aid of masking tools like tape, which can bleed or peel. The core challenge is to transfer paint evenly into the corner’s recess while feathering it out onto the adjacent flat surfaces without creating a buildup that will sag or flash.
The tools are as important as the technique. For most interior corners, a high-quality angled sash brush, typically 2 to 2.5 inches wide, is indispensable. The angled bristles allow precise control to cut into the tight angle. For broader exterior corners, a similar angled brush or a high-density straight brush may be used. The paint itself should be properly mixed and of a suitable viscosity; overly thick paint will drag and leave brush strokes, while overly thin paint will run. Before any brush touches the corner, proper surface preparation—including cleaning, sanding, and applying a suitable primer to sealed or repaired areas—is non-negotiable for adhesion and a uniform final appearance.
Cutting In and Painting Inside Corners
The process for an inside corner begins with “cutting in.” Load the angled brush by dipping about one-third of the bristle length into the paint, then tap (do not wipe) each side of the brush against the inside rim of the can to remove excess. This prevents drips. Start at the top of the corner where the wall meets the ceiling. Position the brush so the angled tip points directly into the corner. Using the narrow edge of the brush, gently draw a line of paint about 2 to 3 inches out from the corner along the ceiling line. Use steady, confident pressure and let the brush’s shape do the work.
Once the ceiling line is established, address the vertical corner itself. Again, point the brush tip into the angle. Apply the paint in a series of downward, vertical strokes, starting from the top. For a right-handed painter working on a corner to their right, it’s often easier to pull strokes downward from the ceiling. For a corner to the left, pushing upward from the floor may feel more natural. The goal is to lay a band of paint that is wider in the corner itself (about 2-3 inches) and feathers out to a thinner layer on the flat wall surfaces. After applying 3-4 vertical strokes, immediately use a mostly dry brush to lightly “feather” or blend the outer edges of this band horizontally onto the flat wall. This blending is crucial to prevent a visible ridge of paint when you later roll the main wall area.
A key principle is to maintain a “wet edge.” Work in manageable sections—about 3 to 4 feet vertically at a time—and complete the cutting in and feathering for that section before moving down. Do not allow the painted edge to dry before blending, as this will cause lap marks. If the corner is long, work in a “step-down” pattern, always blending the new wet paint into the previously feathered edge.
Handling Outside Corners and Edge Control
Outside corners are more prone to damage and require a technique that ensures full coverage on both protruding edges without drips. The approach is similar but requires attention to both faces. Start by painting one of the two walls that form the corner, extending the paint about 2-3 inches around the corner onto the adjacent wall. Use the angled brush to carefully draw a straight line along the very edge of the corner. It can be helpful to sight down the corner from above to ensure your line is straight.
After painting one side, immediately switch to the other face of the corner. The paint you wrapped around from the first side provides a guide. Paint the second face, again using the brush tip to define the sharp edge, and blend the new paint into the wrapped-around section. Because paint tends to pull away from a sharp edge, a slightly heavier load on the brush may be necessary when painting directly over the corner’s apex to ensure it is fully coated. However, be vigilant for drips forming on the underside of the corner. After painting, inspect the corner from a low angle with a light to spot any missed areas or drips, which should be smoothed out immediately with a lightly loaded, almost dry brush.
For both inside and outside corners, the final integration with the rolled wall sections is critical. When you later use a roller to paint the main wall area, roll the paint up to, and slightly into, the previously brushed corner. The roller nap should gently kiss the feathered edge from the brushwork. This merges the textures and ensures uniform sheen and color density, making the brushed corner disappear into the larger wall.
Brush Care and Troubleshooting Common Issues
Maintaining your brush is essential for corner work. Clean brushes thoroughly immediately after use. For water-based paints, use warm water and soap; for oil-based, use the appropriate solvent. Reshape the bristles and let the brush dry suspended by its handle. A brush with splayed or stiff bristles will ruin your cutting-in lines.
Several common problems have specific remedies. If you see brush marks or ridges in the corner, the paint was likely applied too thickly or brushed out after it had begun to set. Always feather the edges while the paint is fully wet. If paint is dripping down an inside corner (a “corner run”), you have overloaded the brush. Unload more paint before starting and use lighter pressure. For a fuzzy or uneven line on an outside corner, the brush may be too worn, or you may be applying paint with a sideways motion instead of drawing the brush directly along the edge. Finally, if the finished corner shows a color or sheen difference from the rolled wall (a phenomenon called “flashing”), it is often because the brushing and rolling were done too far apart in time, allowing the brushed section to dry before being integrated. The solution is to maintain a wet edge and always roll the wall immediately after cutting in the corners for that section.