Washi tape or artist's tape is used to secure your paper to your surface to prevent warping while you paint. These tapes and masking fluid can be used to create borders, frames and designs on watercolor paper.
I recommend using a minimum of #140lb weight, cold-pressed watercolor paper for watercolor painting. If you're not purchasing a "block" of paper, be sure to use washi tape or artist's tape to secure your paper to your surface to prevent warping.
Watercolor Block #140
Paper is secured on all four sides to prevent warping while painting.
What's the difference between a "canvas board" and a "stretched canvas"? A canvas board is most likely cotton canvas glued to a hard piece of paperboard or cardboard and primed with gesso, a sort of sealer. You'll find that canvas board is a bit less expensive than stretched canvas, so it is great for practicing. To display a canvas board painting attach a stick on hanger thingy to the back. (yes, that's an artist's term).
Stretched canvas is typically linen or cotton stretched over a rectangluar or square wooden frame and is then gessoed (sealed) to prep it for painting. It is typically a more expensive canvas than a canvas board. The more a canvas is prepped, the higher the cost.