Your choice of paper affects the results you get from your painting, because it affects the way the paint behaves on the paper surface, so it is important to try various different types of paper, and look at the results to see which one gives you the results you are looking for.
You can, of course, use any type of paper, so you may be tempted to use cheaper paper from a sketch book or even from the back of some spare wallpaper, or office paper, to start with. The problem with this is that the absorbency of these papers varies, too high absorbency and the liquid will be sucked in like ink on blotting paper, too little absorbency can result in the paint running over the surface of the paper in a hard to control way. For this reason it is better to use paper that has been specially made for watercolours.
Watercolour paper has a surface texture, which is called the ‘tooth’. It is this ‘tooth’ that reacts with the brushes and watercolour paint to give the well known ‘watercolour effect’, which is one of the attractions of this medium. Watercolour paper also has just the right amount of absorbency so that the colours remain manageable, and you can get the result you want, with a little bit of practise.
All watercolour papers are made in a choice of three surface textures, the heaviest texture is called ‘Rough’, the medium surface texture is ‘Not’ (sometimes called Cold Pressed) and the smoothest is referred to as Hot Pressed (HP). Rough textured paper is usually considered to be most suitable for larger paintings especially where you use larger, dynamic brush strokes, as it creates a very expressive effect. The HP paper is very smooth and, as this can make the paint more difficult to control, it is probably not the best choice for a beginner, whereas the Not finish suits most purposes and is the one most commonly used by beginners and experienced artists alike.
As well as the surface tooth of the paper there is also the weight, or thickness, of the paper to consider. The weight of the paper is calculated by how much a ream weighs and this can be up to 300lb, which is the heaviest paper. Most people use a little lighter paper, for instance 140 lbs is a common choice. One thing to remember, though, is that the lighter the paper the more it will tend to buckle when wet, so lighter papers will need to be stretched so that they end up flat when the painting is finished. The 300 lb paper can be used without stretching, which you might find is an advantage, but of course the heavier paper costs more, so it’s a matter of finding the best balance of weight verses cost for your own requirements.
Probably the best thing to do is to buy several sheets of quarter imperial paper in different weights and textures, and try them to see which you like the best. Once you have made a choice buying in bulk can save you money and using the same paper for all your work means you will become very proficient in understanding the interactions between the paper, the brushes and the paints, so you will soon be producing watercolour paintings that give you pleasure to create.