What to do with watercolour paper with deteriorated sizing

Watercolour paper that has been kept for too long can have deteriorated sizing, and damage the paper to affect how paint and water behaves.

Sizing for watercolour paper does not refer to the physical paper size, but to the substance that's applied to the paper to affect paper absorbency. Paper without sizing applied will just have water soak through. Sizing is what turns paper into watercolour paper.

Traditional sizing uses gelatin, but nowadays there are also plant-based, synthetic sizing or vegan-friendly sizing are available. Sizing can be done internally, externally or both.

The most important thing to know is sizing can deteriorate when exposed to humidity. In other words, try not to stock up on watercolour paper when you know you don't paint often, or you can't use up watercolour paper that fast.

When the sizing deteriorates, paint would just sink and not move, and this make it difficult to paint. The paper will also create ugly texture, as shown above, and more examples here.

Restoring the sizing

There are products that can add sizing back to watercolour paper, such as the Daniel Smith Watercolor Ground, but they will not be able to restore the paper back to original quality.

Repurpose damaged watercolour paper

So once the watercolour paper is damaged, it's damaged, and the next best thing to do is probably repurpose the paper for something else.

Without the sizing, liquid ink may feather or lines may appear thicker than usual on the paper. Fountain pens and brush pens that use liquid ink are affected. Pens with gel ink work better.

Alcohol markers will feather.

I've tested fineliners with pigmented inks and they work fine, and lines can still appear solid with expected thickness.

Dry media such as coloured pencils, pastels, crayons should work well, affected only by the hotpress or coldpress texture.

Acrylic or paint markers (left) work fine. Gouache (right) can work but more paint is needed to cover the weird paper texture that appears with water.

Shown on the left is diluted gouache and the weird paper texture can be seen. On the right is concentrated paint and the paper texture cannot be seen.

Watercolour paper is expensive. Try not to buy more paper than you can use. Don't stock up even when there's a good sale unless you know for certain that you can use the paper up quickly. If you live in a dry climate, watercolour paper can last longer, but if you live in a humid climate, don't buy too much watercolour paper.