Sizing in watercolour paper is what prevents paint from sinking straight into the fibres the moment your brush touches the surface.
Without it, the paper behaves like blotting paper: pigment disappears, edges spread without control, and any attempt to lift or rework a passage is futile. Understanding how sizing works, and what happens when it fails, is one of the most practical things a painter can know about their materials.
At a glance
Sizing controls how paint sits on the surface. Without sizing, watercolour soaks straight into the paper fibres and cannot be worked or lifted. Sizing creates the layer that lets you control the paint.
External sizing is what most premium papers use. Arches applies gelatin sizing to the finished sheet surface. This creates a consistent layer that responds predictably to wet paint.
Sizing can be damaged by repeated scrubbing. Every time you lift or rework an area heavily, you risk breaking down the surface sizing in that spot. The paper becomes more absorbent locally, and the next wash behaves differently.
Age and humidity degrade sizing over time. Old paper stored in damp conditions may have partially degraded sizing. It will behave inconsistently. Buy recent stock from a reliable supplier.
The wet test reveals sizing condition quickly. Wet a small corner with clean water. If it sinks in immediately and the surface feels rough or fibrous, the sizing has broken down. Good paper resists the water briefly before absorbing it.
What sizing actually does to the paper surface
Paper, by nature, is absorbent. It is made from fibres, and fibres drink water. Left untreated, watercolour paper would draw paint down into its structure immediately, leaving no time to work a wash, no ability to blend, and no prospect of lifting pigment back out.
Sizing is the treatment that changes this. Applied either during manufacture or to the finished sheet, it reduces absorbency to a level that makes the surface workable. Paint sits partly on the surface rather than plunging through it. This is what gives you time, control, and the ability to revisit a passage while the paint is still wet.
The degree of sizing determines a great deal about how a paper feels to paint on. A heavily sized paper holds water on the surface longer and allows more reworking. A lightly sized paper is more absorbent and will not permit the same degree of lifting or repeated layering.
The difference between internal and external sizing
Sizing in watercolour paper is applied in one of two ways, or sometimes both.
Internal sizing is added to the paper pulp during manufacture, before the sheet is formed. It reduces absorbency throughout the body of the paper, not just at the surface. This gives the whole sheet a degree of water resistance, which becomes relevant when a surface is damaged by scrubbing or repeated wetting.
External sizing is applied to the finished sheet as a surface treatment. It forms a layer that further reduces absorbency at the point of contact and also affects how the surface handles the brush, how cleanly edges form, and how well paint lifts. Most premium papers use external sizing in addition to internal sizing, or external sizing alone.
Arches Aquarelle is sized to the core with natural gelatin and also sized externally. The gelatin is applied to the finished sheet and is central to the paper’s character. Many other manufacturers use synthetic agents such as alkyl ketene dimer or PVA in place of gelatin, and some market these explicitly as animal-free alternatives. The handling differences between gelatin and synthetic sizing are real but vary by paper and by technique, and the evidence does not support any categorical claim that one is universally better than the other.
How sizing affects paint behaviour, lifting, and edges
Sizing is the reason wet-on-wet painting works. When you drop a second colour into a wet wash on a well-sized paper, the paint spreads in a controlled way because the water is sitting partly on the surface rather than being drawn down into the fibres. The time available to work the passage is determined largely by how the sizing holds that moisture.
Lifting is similarly dependent on sizing. When pigment has not bonded deeply with the fibres, a damp brush or clean sponge can remove it. On a paper with strong external sizing, this works reliably. On a more absorbent paper, or on a spot where the sizing has been damaged by previous scrubbing, the same technique removes far less pigment.
Edge control follows the same logic. A hard edge forms when paint dries at a boundary before it can spread further. The rate at which the surface dries is influenced directly by how the sizing holds moisture. Predictable sizing means more predictable edges.
What causes sizing to break down
Sizing is not permanent. Several things accelerate its deterioration.
Age is the most straightforward cause. Older stock, particularly paper that has been sitting in storage for a long time, may have partially degraded sizing. This is one practical reason to buy from a supplier with regular turnover. The UK watercolour suppliers listed on this site are worth checking for freshness and storage conditions.
Humidity is another significant factor. Paper stored in damp conditions absorbs moisture, and prolonged exposure compromises the sizing layer. Paper stored in a stable, dry environment ages much more slowly.
Repeated scrubbing and heavy lifting during painting damages external sizing locally. Each time you work aggressively into a passage, you degrade the surface layer in that area. The paper becomes more absorbent at that spot, and subsequent washes behave differently from the surrounding surface. This is not a failure of the paper so much as a consequence of how it is being used.
Heavy initial wetting, particularly when stretching paper by soaking it in a bath, can also affect external sizing on some papers. This varies by paper and is worth testing before relying on a particular method.
How to tell if the sizing on a sheet has degraded
The simplest test is also the most reliable. Take a small corner of the sheet and wet it with clean water from a brush. Watch what happens.
On a well-sized sheet, the water will sit on the surface briefly, forming a soft bead, before being absorbed. The surface will feel smooth under the brush.
On a sheet with degraded sizing, the water sinks in immediately. There is no brief resistance. The surface may feel slightly rough or fibrous rather than smooth. Paint applied to this surface will spread unpredictably, colours will look muted and dull because pigment is being drawn deeper into the fibres, and lifting will be weak.
If degradation is confined to the edges of a sheet, that is common with older or poorly stored stock. The interior of the sheet may still be usable, but test before committing a painting to it.
What to buy
Two papers consistently deliver reliable sizing performance and are straightforward to buy in the UK.
Arches
Aquarelle 300gsm Cold Press
Arches uses natural gelatin sizing applied externally to the finished sheet. The result is a surface that responds predictably to wet paint, allows confident lifting, and maintains consistent edge behaviour across the sheet. Batch-to-batch consistency is notably reliable, which matters when you are learning to read how a specific paper behaves. Available in sheets, pads, and blocks from Jackson’s Art Supplies and Cass Art. The most authoritative paper for anyone whose work depends on controlled wet-on-wet passages or repeated lifting.
Fabriano
Artistico 300gsm Cold Press
A well-sized alternative at a lower price per sheet. Slightly different surface character from Arches, with a texture some painters prefer for certain techniques, but similarly reliable sizing consistency. A reasonable choice for painters who want good sizing performance without the cost of Arches for every session. Also available from Jackson’s Art Supplies and Cass Art.
Where to buy
Jackson's
Best overall range
Cass Art
Useful mainstream stockist
Amazon
Fast, broad availability
For a broader comparison of how these two papers perform across different working methods, see the materials section of this site.
The practical verdict
Sizing is not a secondary consideration. It is what makes watercolour paper capable of supporting the techniques that define the medium. A well-sized sheet holds water, accepts glazes, permits lifting, and forms edges with some predictability. A poorly sized sheet, whether through degradation, poor storage, or light original treatment, resists none of these things.
The practical upshot is simple. Buy from a reliable supplier with regular stock turnover. Test older paper before trusting it. Avoid aggressive scrubbing in areas where you plan to glaze again. And if a sheet behaves strangely, check the sizing before assuming the fault lies with your technique.