I've been asked several times if I could explain clearly how to stretch watercolour paper and so here are several photos showing my (reasonably foolproof) method of stretching 140lb Saunders Waterford NOT watercolour paper.
This is my board - it is made of ¼" plywood and I use each side alternately so that by warping slightly in different directions each time, it remains flat. By the way, the tape around the edges is just where I haven't bothered to remove it after I had cut out my previous painting - I generally leave it a few times before removing it all.
Here is my roll of gummed tape and the stips cut from it. Cut this tape before you handle your paper so that your hands are totally dry as you don't want any of the glue removed from it. Make sure to make them slightly longer than the edges of your paper.
I take my paper (I normally work on a half full sheet) and soak it in a clean bath (no traces of cleaners or other chemicals) for approximately 15 minutes. I've been known to forget it and leave it in there for more than an hour and I've never suffered adverse consequences!
Remove the paper from the bath with clean hands, holding it along one edge and let the excess water drip off. Take it to your board and lay it down.
With clean kitchen roll - just a couple of sheets - remove the excess water. Just lightly run your hand over the back of the kitchen roll - no heavy handedness.
Then just lighly run the kitchen roll down the edges of the paper.
Now wet half of a piece of tape. The reason for leaving half of it dry is as it absorbs the water from your paper, if it is already wet, it becomes too wet and won't stick. Do just completely wet the very ends of the tape though as they will extend further than your paper and won't otherwise stick down.
This photo shows one piece of tape stuck down. Run the back of your nail up and down it and you will see it start to adhere to your paper - do this until it is smooth.
And the final result ... a lovely taut, clean piece of paper just waiting to be painted on! :)
I hope you have enjoyed this tutorial and if you have any questions, just leave a comment or email me at firstname.lastname@example.org