You can paint oil or acrylic paint on almost any surface but watercolour must have its own specially constructed paper, which isn’t really a paper, it’s closer to a kind of fabric.
Watercolour paper is made from linen and/or cotton fibres which are slopped back and forth in linear vats till they ‘felt’ together .The difference between this and cartridge paper is that ordinary day to day paper is made out of wood pulp and is intensely acid, it will therefore yellow and crisp in a matter of years and disintegrate.
Watercolour paper generally possesses these characteristics – it has a textured surface which can be evenly random ( vat made sheets) or uniform ( machine rolled). This texture varies from extremely smooth – Hot Pressed papers – to medium textured to very rough (called Torchon). It comes in very thin to very thick sheets - called lightweight, medium to heavyweight which is designated by the gram or pound weight eg: 300gsm, or 140lb. This is the weight of a whole ream of paper, not the one sheet! And it ranges in colour from very bright white, to cream to even greyish or bluish.
Hot Pressed papers are silken smooth ‘ – they present little resistance to the brush stroke, make colours look quite bright and make it difficult to achieve even washes over large areas without a great deal of practice and experience. Good for fine detail work they come from thin and flimsy to quite thick and are priced (as are all papers) with the heavier papers being the most expensive. Because they are very smooth surfaced they absorb brushstrokes very fast leaving you only a tiny window of opportunity for blending your strokes.
‘Cold Press’ papers have texture which can vary from medium to very rough and practically turn into board in the heavier weights The texture in a Cold Press paper slows down the rate of absorption, so you have more time to blend edges..
The Rough papers have a surface which catches at the wash giving you interesting textures. It is harder to sketch and depict details on super rough papers.
If you are just starting out, go for something in the middle, a medium weight paper with a cold pressed surface (around 300gsm). You can make a more informed choice about which paper to work on when you have a better idea of what sort of subjects you want to paint. Botanical paintings for instance are easier on a smooth paper, a seascape will look great with a rough paper but these are just guidelines not rules.
Some of the best results can occasionally happen when totally inappropriate papers are chosen for the subject. It’s important to try out different papers and experiment and have fun. Buying a sheet of paper and cutting it up with sharp scissors is more economical that buying watercolour blocks and is generally better quality paper.
Watercolour paper is vat made, acid free and as a further refinement is ‘sized’ on one side so the wash doesn’t sink straight into the paper. Without the sizing, the paper would be closer to blotting paper. Be aware that many etching and printing papers look like watercolour paper but are not sized in this way. Just remember, the right side of the watercolour paper is the side you can read the watermark on, if you’re seeing the brand name of the paper backwards when you hold the sheet up to the light, you’re working on the back of the paper and the results will less than satisfactory as you are working on the unsized side of the sheet. Double sized sheets are available in some brands, but at double the price!
When you sweep a loaded brush across the surface of watercolour paper, tiny puddles form in the textured depths of the paper, while a thinner layer of colour sits on the ‘ridges’ between. The luminosity of the dried wash depends on the brightness of the paper, the consistency of the sizing, the fineness of the grind of the pigment in your colour, the quality of the pigment itself, the amount of water in the wash mix and the rate at which it all dries undisturbed. Viewed side –on under a microscope, watercolour paper is made up of all those interlocking layers of fibre and acts like a sponge to soak up the water. The speed at which it does this depends on the type of paper so there is no one rule for all types, makes and weights of paper. You must experiment and test out unfamiliar brands of paper. Once a wash has been applied the ‘water’ in the wash sinks through the paper leaving the ‘colour’ on the surface layer where evaporation will gradually dry it out. But those lower layers are still holding a lot of water, so you have to wait until they dry as well which is why paper can look dry but still feel damp to the back of the hand.
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