Generally watercolour painting requires fairly special paper made by a number of companies. Fabriano, Arches, Daler Rowney (Langton), Saunders-Waterford, Bockingford and Hahnemuhle are common brands used in New Zealand.
High quality watercolour paper is generally made of linen or cotton rag, is acid free, resilient and is coated with size (some sort of gelatin, starch mix) which helps the paint flow on the paper without being absorbed into the fibre.
Paper comes in various sizes, weights, colours (although usually white or cream) and surfaces, in terms of size you can buy gummed blocks from approximately A6 (148 x 105mm) up to A2 (594 x420mm) although “A” sizes are not common and the sizes seem to be randomly chosen, perhaps to provide golden ratio sizes. Larger sizes are available in sheets and rolls.
The weight (thickness) of paper determines how much the paper will “deckle” or bulge when wet. Commonly available weights are 150, 300, and 600 gram/m2 .
Surfaces available are “hot pressed” (smooth) “cold pressed or not” “(medium) and “rough”.
Gummed blocks, where the paper is gummed on all sides to hold the block together to reduce deckling are satisfactory up to about A3 size and are good for most users who do not work too wet. For larger sizes I prefer to stretch sheet paper onto a board a process I will explain later in this tutorial.
For most uses I would recommend 300gram weight although a heavier paper may be better for larger works and can be used without stretching. I find 150gram paper too light as it will bulge badly when wet except perhaps in very small sizes.
I mostly use cold pressed (not) paper, which has enough grain for my work. Hot pressed paper is good for line and wash as the smooth surface is easier to draw on with a pen but it is more difficult to apply a wash to. Rough paper is great for achieving granulation and for using “dry brush” to produce textural effects.