I recently bought a large roll of Arches 140lb cold press watercolor paper and for the first time, am stretching my paper. Typically, I purchase 22″ x 30″ sheets which are packaged flat and may be simply taped down and minimally buckle (a side note here- I iron my paintings before I flatten them before framing). The rolled paper requires more handling. Here’s my current method, working with 40″ x 30″ sheets:

First, I have a large piece of masonite board that is slightly larger than the painting I’m working on. Next, I begin to unroll the paper, wetting it with a large 5″ varnish brush or with a clean sponge. As I wet the paper, I wet the board as well and the two surfaces stick together. I carefully unroll more of the paper, continuing to wet both sides of the sheet and my work surface. When I finally reach the edge of my support board, I cut the sheet from the roll using a razor blade. Now, I continue to wet the paper, working out the bubbles until the whole surface is shiny-wet and completely smooth. Next I use a gummed paper tape to secure the paper to the board and allow it to completely dry before sketching out my painting. Instead of tape, other artists have recommended stapling the paper to my board which I may try in the future. Let me know if you have any questions or other experiences working with large watercolors!