Paper Weight
The next consideration is the paper thickness. This is worked out basically on how much a ream of 500 sheets weigh and can be quoted in lbs or grams per square metre. 140 lbs (300gsm) is a good weight to start with. The heavier papers will be less inclined to cockle or warp when painting but the thinner ones, 90lbs, will need to be stretched before working on.
To stretch watercolour paper you first soak it in a tray or a bath of water for several minutes (the time required may vary and only practice will produce the right result) then place the paper flat on a drawing board and stick the edges down with gum paper (and or staples) and it should dry out as tight as a drum. Papers are also made in ready prepared stretched blocks, where all you have to do is to carefully release it from the block after the painting is finished.
