Sizing is a treatment used to control how readily paper takes up water. It is separate from the sheet’s dimensions, weight and surface texture. When two watercolor papers accept a wash differently, the sizing is one possible part of the explanation; a single droplet does not prove the formula or diagnose damage.
Internal sizing and surface sizing
St Cuthberts Mill describes internal sizing as an agent added while the paper is still a pulp mixture. Its purpose is to help control absorbency for the intended use. The mill deliberately makes different ranges with different resistance to water: “more absorbent” is not automatically the same as “defective.”
Surface sizing describes treatment of the formed sheet. For example, Fabriano describes surface sizing for Artistico, while ARCHES describes gelatin sizing through Aquarelle paper. These are product descriptions, not evidence that all papers use the same recipe, amount or process.
What does sizing change for a painter?
A watercolor surface needs to accept water in a way suited to the work. The St Cuthberts explanation connects absorbency with the time available to manipulate wet paint. It also uses the terms hard sized for greater resistance and soft sized for greater absorbency. Neither term means the paper is physically thick or thin.
Keep these questions separate when choosing a sample:
| Label | Question it helps answer |
|---|---|
| Dimensions | Will the sheet fit my planned image and support? |
| Gsm | What is the nominal mass per square meter? |
| Surface | What texture is specified? |
| Sizing | What treatment or absorbency guidance does the maker describe? |
Internal vs external sizing at a glance
| Term | What it describes | What to check |
|---|---|---|
| Internal sizing | An agent introduced into the pulp during papermaking | The maker’s absorbency guidance for the range |
| External or surface sizing | Treatment applied to the formed sheet | The stated treatment and preparation instructions |
| Hard or soft sized | Resistance to water rather than sheet thickness | How long a wash remains workable for your technique |
These terms describe different aspects of paper; they are not a best-to-worst ranking. For a painter, the useful question is whether a wash gives you enough time to blend before it absorbs.
Can a home check tell you the sizing has failed?
It can give you an observation to investigate, not a conclusive diagnosis. A drop that disappears quickly does not identify the treatment, prove an entire batch is faulty or establish why that sample behaves that way. The mill’s absorbency guide describes a simple drop observation, but it also uses controlled quality tests during manufacture.
If a familiar paper suddenly behaves differently, record the range, batch information if available, side used, storage, preparation and tools. Compare an untouched sample using the same paint and water. Photograph the label and the result if you need to ask the maker for help. Keep “what I saw” separate from “what caused it.”
If a dried color will not lift, also check whether the watercolor is staining. If your concern is a speckled wash, compare it with pigment granulation. Neither observation alone establishes a sizing defect.
Check handling before trying to repair the paper
St Cuthberts warns that detergent contamination during soaking and excessively long soaking can compromise a paper’s intended behavior. That is a reason to follow the appropriate preparation instructions and keep the soaking equipment clean, not a recipe for diagnosing every uneven wash.
This guide does not recommend adding homemade sizing to a valued painting or washing a paper to “improve” it. If you want a different working response, selecting another documented paper and running a small exercise is easier to interpret than changing an unknown treatment yourself.
Continue with composition comparisons or choosing a side, depending on which question your label leaves unanswered.
Sources checked September 9, 2026. Research-based explanation and original conceptual diagram. No chemical analysis or paper-defect test was performed.