This is a warning to those who likes to stock up on paper and those who buy too many sketchbooks but can't use them fast enough.

Watercolour can deteriorate over time. More specifically, the sizing of the watercolour paper can deteriorate.

Most watercolour paper are sized with gelatin either internally or externally. The sizing allows water or paint to sit on the surface of the paper rather than soak into the paper fiber.

If watercolour paper is exposed to humidity, the sizing can deteriorate, making the watercolour paper "normal" paper again, so to speak. By deteriorate, I mean the sizing would dissolve and disappear. When there is no more sizing, water and paint would be absorb by the paper and colours would look off and paint would have difficulty moving around, proper wet on wet techniques will be difficult to achieve.

Should above are pages from a sketchbook I got custom made a long time ago. It wasn't used for a long period of time and the sizing had deteriorated. When I painted on the paper, it looked weird, as if the paper fiber is becoming more obvious. The paper is Arches Hotpress 100% cotton watercolour paper.

Here are two more pages from the same sketchbook.

Again the nasty paper texture can be seen.

On this particular page, the sizing at the area where the road is has deteriorate but the area where the motorcycles are still looks fine.

This is another custom made sketchbook. This was bound with Fabriano Artistico coldpress 100% cotton watercolour paper. This sketchbook also wasn't used for a long period of time and the sizing had gone bad. Here in Singapore, humidity levels are high so that's a major source of problem. Sometimes the covers of my sketchbooks would also grow mould because of the humidity.

Colours appear washed out and the paper fiber texture is noticeable.

With good watercolour paper, paint is supposed to sit on the surface of the paper. Light goes through the transparent paint, gets reflected off the paper, and goes through the paint again before it reaches our eyes. In short, the paper is actually lighting the paint from behind. When the paint gets soaked into the paper fiber, the paper no longer lights the paint because there's no paint on the surface, and hence the colours appear dull.

Here's a close up on thr top right area.

Here's a watercolour sketch on a sketchbook recently made with Lanaquarrelle 100% cotton coldpress watercolour paper. Colours are vibrant and look as expected.

Here's a watercolour sketch on Arches 100% cotton hotpress watercolour paper when the paper still had good sizing.

The colours appear very nicely on the surface and there's no nasty paper fiber texture. And it's very easy to use wet on wet techniques on good paper too because the paint moves around easily on the surface. But once the paint gets soaked into the paper, they are not going to move much.

After you have painted on the paper, whether the sizing deteriorate is no longer important, because once the watercolour paint it dry, it's permanent. Dry watercolour paint is not going to move and soak into the paper.

So this is my warning to those who may stock up on paper, maybe during a sale, or buy too many watercolour sketchbooks that you can fill up fast enough. Don't make the same mistake as I have made.