1. Machine-made watercolor papers come in three surfaces: rough, hot-pressed or HP, and cold-pressed (or NOT).

2. Rough watercolor paper has a prominent tooth, or textured surface. This creates a grainy effect as pools of water collect in the indentations in the paper.

3. Hot-pressed watercolor paper has a fine-grained, smooth surface, with almost no tooth. Paint dries very quickly on it. This makes it ideal for large, even washes of color.

4. Cold-pressed watercolor paper has a slightly textured surface, somewhere in between rough and hot-pressed paper. It's the paper used most often by watercolor artists.

5. Watercolor paper differs from manufacturer to manufacturer, so experiment not only with the different kinds of paper but also with various brands of paper.

6. The thickness of watercolor paper is indicated by its weight, measured either in grams per square metre (gsm) or pounds per ream (lb).

7. The standard machine weights are 190 gsm (90 lb), 300 gsm (140 lb), 356 gsm (260 lb), and 638 gsm (300 lb). Paper less than 356 gsm (260 lb) should stretched before use, otherwise it's likely to warp.

Tips:
1. Watercolor paper is usually white, but it need not be. A variety of cool and warm tints is available.

2. Use acid-free paper for paintings you wish to keep as this will yellow less with age.

3. Cold-pressed watercolor paper is called NOT paper because it's not hot-pressed.

4. You can buy prestretched watercolor paper in drawing blocks. When you've finished a painting, you use a palette knife to remove the top sheet from the block.