The thing that makes watercolour paper watercolour paper is actually the sizing it has. When it comes to making watercolour paper, sizing means to cover, stiffen, or glaze the paper either internally or externally with gelatin so that the paper can handle water and watercolour paint. In simpler terms, sizing is the treatment process of the paper to allow the paper to work with water. Gelatin or gelatine is a translucent and colorless compound derived from collagen taken from animal body parts.
When you apply water onto paper without sizing, the water would just soak through to the other side. The paper will become soft and easy to tear. Painting on such paper often results in the paper fiber coming off the paper's surface.
Watercolour paper with proper sizing allows the paper to absorb some water but most of the water and paint will remain on the paper. This protects the paper from being damaged and allows the paint to move around on the wet surface.
How thick the paper is and what the paper is made of is not as important as having proper sizing.
Here are some pieces of paper I've tested.
This is 190gsm 100% cotton paper in the Strathmore Mixed Media 500 series journal. The paper quality is excellent, but because it's not sized, it's actually not suitable for watercolour usage. If you use too much water, the water will soak through the page. When you lay paint, the paint will also get absorb quickly into the paper and won't move much. As a result, it's incredible difficult to use wet on wet techniques to paint gradations or blend colours. Yes, you can still use wet on wet but it's extremely challenging.
Lifting paint off that Strathmore paper is also difficult because the paint will be absorbed into the paper.
This is 180gsm Bristol paper, a type of smooth paper made for pen and ink illustration. This paper is thin and will buckle significantly. It's not sized so the water will soak through, and paint will not move much once it's on the paper so expect hard edges.
This is Fabriano Studio watercolour paper with 25% cotton. This paper has proper sizing and is wonderful to paint on. However, it still has a tendency to absorb paint easily so wet on wet techniques may still be challenging.
Canson XL watercolour paper is wood pulp paper and is sized to handle watercolour. Wet on wet is extremely challenging on this paper because once the paint is on it, it's not going to move or blend much.
This is Strathmore writing paper (90gsm) with 25% cotton content. This paper is thin and not sized. Applying water on it soaks right through quickly and paper buckles straightaway.
That's the water and the paint soaking through to the page behind.
Daler Rowney's Aquafine paper is also wood pulp paper. Compared to the Canson XL watercolour paper, wet on wet techniques is still achievable on this paper, but it's not as easy compared to high quality cotton watercolour paper.
You can lift paint off the Aquafine paper quite easily.
This Arches 100% cotton watercolour paper has fantastic sizing. It absorbs just the right amount of water to allow the paint to remain fluid and spread around easily. This is great for wet on wet techniques. Painting gradations and colour blends is easy.
Notice the white portion of the paper on the right side? That was actually painted with French Ultramarine but the paint was lifted off, almost completely. It's easy to lift the paint. I just had to add water, use tissue to lift the paint off. I repeated the process a few times and the paper almost looks new.
Whether or not lifting paint is successful also depends on the paint you use. French Ultramarine is not staining and hence easy to lift. Paint like Phthalo colours that are made with tiny particles will stain and go into the paper which makes them difficult to lift.
Proper sizing is important also because it affects the vibrancy of the colours.
The way we see colours work this way. The light will shine through the transparent watercolour and light up the paper surface. The light from the paper will then reflect back through the watercolour again so that we can see the colours.
So if you're viewing your painting in a dim room, there won't be much light hitting the paper, and hence the colours will appear dull.
So on paper with proper size, the watercolour will be on the surface, and the bright paper will shine and light up the colours on it.
For paper without sizing, most of the watercolour will be absorbed into the paper. So since there isn't much watercolour on the surface, even if the paper is bright white, it's not lighting up much colours. That's way the colours will appear dull.
You have to test your watercolour paper in order to find out whether it has proper sizing. Or you can just go with reputable brands such as Arches or Fabriano Artistico paper that you know you can depend on.
Painting on paper with lousy sizing or inadequate sizing, which is the case with many student grade watercolour paper, can be frustrating. It's difficult to get the paint to move fluidly and freely. When I started learning watercolour, I remember being confused as to why I wasn't able to blend colours even though I had followed the exact instructions from books. The reason was the paper I used just wasn't able to perform because of the sizing it had.
When it comes to painting with watercolour, you need paint, paper and brush. The paper is actually the most important component, followed by paint and then the brush. Good watercolour paper is expensive for a reason.
To learn more about watercolour paper and its characteristics, check out this article on Jackson's Art.