Watercolor paper sizing

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All watercolor papers are sized, traditionally with gelatin, but nowadays sometimes with a synthetic sizing agent. The sizing has a crucial impact on how watercolor paint behaves on the paper and is an aspect of a paper’s quality that should be considered by all watercolor painters.

The purpose of sizing is to prevent watercolor paint from being immediately absorbed by the paper. A watercolor paper without sizing is impossible to paint on. The paint needs some time to remain on the surface before being gradually absorbed. A heavily sized paper allows for slow absorption, while a lightly sized paper absorbs the paint quickly, creating an entirely different type of paint layer.

There are two types of sizing: internal and external. All papers are sized within the pulp, known as internal sizing. Some papers are also sized on the surface, called external sizing. It is usually expensive watercolor paper that also has an external sizing, as it makes the paper more expensive.. However, there are exceptions—high-quality papers that lack surface sizing, but the reverse does not exist; There is no cheap paper with external sizing.

A heavily sized paper, especially on the surface, produces bright and clear colors with sharp edges, while a lightly sized paper results in softer contours and weaker colors. Wet-on-wet painting is more effective on a heavily sized paper because the paint remains on the surface longer, whereas on a more absorbent paper, wet-on-wet washes may become patchy or uneven.

On the other hand, layering is much easier on a lightly sized paper since the paint is absorbed into the paper. On a heavily sized paper, paint is easier to lift, making layering less effective. Dried paint layers can be disturbed when painted over, making the technique difficult to manage. A lightly sized paper dries faster than a heavily sized one, which also facilitates layering.

Heavily sized papers are also more resistant to rough treatment. This is not the case with lightly sized papers, which can be damaged if handled too aggressively. If you plan to scrape, rub, sand, or use masking fluid or tape, you should choose a paper that is well-sized. Lightly sized papers may crumble and break apart under such techniques.

Different Techniques and Effects

Drying time: A lightly sized paper dries faster than a heavily sized one.

Wet-on-wet: Heavily sized papers provide smoother transitions since moisture is not absorbed too quickly. A lightly sized paper, which absorbs water faster, often creates more spontaneous patterns but carries the risk of blotchy areas.

Color saturation: The softer the sizing, the paler and “softer” the colors. If you aim for high contrast and distinct colors, choose a heavily sized paper.

Edge definition, granulation, and blooming: Heavily sized papers create sharp edges around painted areas, known as a hard edge. They also enhance granulating colors since the pigment has more time to settle in the wet paint. Blooming effects occur more easily on heavily sized papers; on a softly sized paper, paint is absorbed quickly, making such effects harder to achieve.

Lifting and corrections: Choose a heavily sized paper for easier lifting of dried paint. On a softly sized paper, the paint adheres more firmly to the surface.

Layering: Often problematic on a heavily sized paper since underlying layers tend to lift when overpainted. A lightly sized paper holds layers better and is generally more suitable for this technique.

Sanding and scraping: If you want to use sandpaper, a sharp knife, or similar tools to remove paint for special effects, you should use a heavily sized paper. A softly sized paper struggles to withstand such treatment.

Masking: Tape or masking fluid can be used even on moderately soft papers. However, some papers are so softly sized that they get damaged by both masking tape and masking fluid.

Dry brushing: The paper’s surface texture primarily determines whether a paper is suitable for this technique, but the degree of sizing also affects the result. I find it easier to achieve fine dry brush effects on a heavily sized paper.

Erasing on a finished painting: Have you ever tried to erase a pencil line on a finished painting and failed? This may be because the line was covered by the surface sizing, which is water-soluble. Once dry, it becomes very difficult to remove a pencil sketch. This issue is more common with heavily sized papers.

Navigating the Paper Jungle

It is not easy to find information on how different manufacturers size their papers, as none specify the exact degree of sizing used in a particular watercolor paper. You likely have to experiment to find what suits you best. I have attempted to create an approximate list based on some of the papers I have tested. Not all papers are included, as I have not painted on every watercolor paper available. However, many of the most common high-quality papers are on this list, and if I use a paper in the future that is not included, I will add it. Keep in mind that this is just my personal evaluation of sizing, so it may not be entirely precise but is based on my own judgment.

Heavily Sized Papers

Medium-Sized Papers

Softly Sized Papers

Very Softly Sized Papers