If you want to paint several versions of a drawing or to just save time when transferring your sketches to watercolor paper, I have found that it is easiest to use a printer to transfer a light version of the line drawing onto your paper. Although the drawing will now be permanent and not erasable, once you have filled in an area with paint, the lines disappear and nobody is the wiser. Purists may not like this idea but as long as I am the one who did the original drawing, I feel no guilt about using modern technology to aid my creative process. So here goes:
First you take your sketch and create a line drawing by tracing over the pencil with permanent ink. I like to use a technical pen. Erase the pencil and scan the line drawing into the computer. Format it using Photoshop to the desired size. Now fade the drawing down to 20-30% opacity and print it onto a sheet of watercolor paper.
At this point, it is important to stretch your paper before painting so that it doesn't warp. As I have illustrated in previous posts, I like to use clamps and a non-porous surface such as acrylic, glass or a plastic cutting board. (Wood lifts when wet, cardboard warps.)
Soak your paper in a room-temperature water bath for about 15 minutes. You should see the paper expand somewhat and become soft. The printer ink should not run or fade. Be sure not to handle the paper with your hands very much, especially where the design is. The oil from your fingers can rub onto the paper and present a problem for watercolor.
Lift the paper out of the bath and let the water drip away for a minute or two. Do not brush the paper with your hands to make this process go faster.
Gently lay the paper onto your board and fold the top over the edge, clamping it down every three inches.
Now lift the other end of the page with both hands and gently stretch it toward the opposite end of the board and clamp that down as well. (Clamp the center first, then both ends, then fill in between.)
Now clamp down the sides of the paper - not too close to the design as the clamps will leave pressure marks on the paper. (Please note: my wide format printer is broken at the moment so I had to cut this paper to fit my other narrow printer. This is not ideal because you want to have a good blank border around your design. )
Let your wet contraption sit for a day (or at least overnight) to dry thoroughly. I like to leave it on the same board to paint it, removing all but a few of the clamps to make holding it more comfortable.
Another note: I usually stand my wet paper up to dry. For some reason my cat loves to walk across a horizontal "field" of wet paper. She probably wants to put her artistic stamp on it as well.