Just came upon this discussion and wanted to chime in something I've been doing for the past 6+ months and so far has the best way to "stretch" my watercolor paper.
Longevity of the paper is still to be determined for this untraditional, but very effective way of "stretching" But I've found this method has been the best way to work with my watercolor paper completely flat even if you work with very wet techniques.
I use both Arches and Fabriano 140lb. Cold Press watercolor paper sheets.
1.) I wet my paper on both sides then set it aside.
2.) I then use a wallpaper adhesive
(Roman Pro-543 Universal Wallpaper Adhesive...I'm pretty heavy handed with the stuff and cover beyond the size of my precut watercolor paper I'll be working on.)
I brush the adhesive onto a clean non-porous surface. (I've cut down a large piece of kitchen table laminate and adhered that to a drawing board so it's lays flat and it's sturdy but still light weight.)
But any clean non-porous surface would do just fine.
3.) I place my wet watercolor paper onto the non-porous surface where I brushed the wallpaper adhesive on. I then use a clean flat watercolor brush to press the paper and work all around the papers surface and edges just so I know the paper and the adhesive meet every bit of the underside of the watercolor papers surface.
(Might be an unnecessary step but it's quicker then stapling or pinning or taping from my experience.)
4.) I dab the excess water off and can start painting right away or let it fully dry overnight. (Taping the border where the edge of the paper and the non-porous surface meet is a good idea because if water seeps underneath the adhesive can come undone. But if you're careful or just tape it down you shouldn't have a problem.)
5.) This is the most important step...Removing the watercolor painting from the wallpaper adhesive without damaging or ripping your work after you're done.
I use a piece of Yupo paper a strip that is about 2" longer than the Height of my painting. Hold the strip of Yupo with both my index and thumb and I slide it underneath a corner and slowly work my way to the center and out the other side. Watercolor paper lifts from the adhesive and non-porous surface with ease. I clean the non-porous surface with water and a cloth and repeat.
Hopefully someone finds this helpful as much as I have. Any questions doing this method shoot me a message and I'll gladly help!
One Last Tip: Do some practice pieces before tackling a finished piece. It took me a couple times to get it right but so far worth the effort and is now my primary way of "stretching" my watercolor paper!