This surface is the most commonly used and seems to be the easiest for beginners to learn with. When paper gets wet, it buckles and warps. There are different techniques watercolorists use to combat this problem. The easiest is to use a thicker paper. Watercolor paper typically comes in weights ranging from 90 lb.
Blocks are watercolor papers that are similar in form to a pad.
Instead of only the top being sealed, all four sides are sealed on a watercolor block, leaving only a small corner open. The painting is done directly on the block and left to dry. When the painting is completely dry, the seal holding the top page on can be broken by inserting a dull knife in the open corner and sliding it along the edge all the way around. Another way is to simply tape down a sheet of paper all the way around. While the paper is wet it will have waves in it and may make it difficult to control the way a wash may flow.