Always experimenting, I tried a fairly new watercolor paper -Canson's Moulin Du Roy. I expected it to be similar to most of the other papers on the market and used it for a demonstration painting for my Friday watercolor class in Kaneville. It resulted in one of the worst demo paintings that I've ever done and I did not even attempt to complete it. Lesson learned–experiment with new materials on my own time.
Watercolor paper comes in different types of surfaces. Hot pressed-smooth, cold press-slight texture and rough-very textured. The choice of the type of paper you use is determined by your approach to a painting. Hot pressed paper is great for painting very precisely. Cold and rough are great for more abstract or softer paintings.
The other factor in the choice of paper is the sizing. With the exception of rice paper, all watercolor paper is manufactured with a sizing (glue) in the paper, which is referred to as internal sizing. The sizing controls how much the color flows on and into the paper. Some papers add more sizing to the paper after it is dry. This is called surface sizing.
The surface sizing allows you to put a "wash", (water plus pigment) on the paper which does not spread. This enables the artist to do a more precise painting. The disadvantage is that you lose the ability to apply different layers, which is I believe the most important property of transparent watercolor.
When light passes through each layer, it refracts or bends causing it to vibrate, which makes the colors appear more intense. The surface sizing keeps the pigment on the surface. This limits the layering of additional washes.
Another advantage of either the hot pressed paper or paper with a surface sizing is that you can "lift" some of the pigment from your paper. Because the pigment is on the surface, some can be lifted by rewetting an area and then blotting or scrubbing to lighten the value.
So, what was so different about the Moulin Du Roy paper? Canson's description cites that: "internal and surface sizing permit the lifting of dried colour and the opportunity to refine and rework the watercolour..." However, when I applied the first wash, it was absorbed immediately into the paper, which would indicate that it did not have a surface size. And, to my horror, fifty percent of the intensity of the color was lost. On the other hand, when I tried to lift pigment in an area, it lifted very easily, which would indicate that, in fact, it did have a surface size. Sadly, the demo was a disaster on the first wash, and it was almost impossible to recover from that point on. (I am grateful for my very compassionate students!)
Here is the studio painting in progress from which I drew the reference for the demo. Without laying down a strong first wash, you will not get the intensity of color upon which the rest of the painting is built.