How to Thin Oil Paint: Fat Over Lean Layering

Mix a fast-dry medium into your darkest color, no more than about 20 to 25 percent of the paint. Push and wiggle a flat brush in the mix so the color works deep into the bristles. Hold the brush almost parallel to the canvas and scrub the paint in. It should look thin and a little translucent. That is your lean layer, and it dries fastest.

To work without solvent, dip the dirty brush in a jar of walnut oil. Pull it out and wipe the bristles hard into a shop towel or rag to get as much paint off as you can. Dip and wipe again until the brush looks fairly clean. Keep the jar of oil. It darkens over time but you can reuse it again and again.

Scrape all the fast-dry medium off your palette with a palette knife and a paper towel. This layer is paint only. Load the brush the same way, then run it over the edge of the paint pile too so a little extra sits on the bristles. Paint right over the thin layer. It should go on slightly thicker, a little fatter than the one below.

The top layers get thicker again, still with no medium. Instead of pushing paint into the bristles, scoop more of it onto the brush. Hold the brush at the same angle as the canvas and lay the paint down gently. Then leave it alone. Going back over it drags up the wet layer underneath. You can soften or blur the edges a little afterward.

The last thick layer is usually your lightest color, often the whites. A palette knife lays it on heavy and clean. Pick up a ridge of paint on the underside of the blade and press it onto the canvas with the knife nearly flat. Most painters start with dark, thin values and finish with light, thick ones like this.

Forgot to leave an area light? Wrap a paper towel around your finger, a brush handle or any small tool. Press it into the wet paint and wipe that spot back toward the canvas. Taking the paint off first means the new color will not drag the layer below into it. You can also wipe out small areas with a clean brush.

Now paint the light color into the area you wiped. It stays bright because there is little wet paint left under it. Compare it with a spot painted straight over wet paint. Some of the lower color mixes in there and the light turns dull. Angela paints two triangles so you can see the difference side by side.

Fine detail work is the one place paint does not get physically thicker. It needs to flow. Add a little walnut oil or linseed oil to the paint so it runs more smoothly off a fine brush. That keeps the layer fat even though it is thin, so it is safe on top of the thick paint. Use it for lines, final highlights and your signature.