I use assembled stretcher bars to stretch my watercolor paper. One of my studio mates back in college ( a long, long time ago) showed me this unique way to stretch watercolor paper and I've been using this method ever since. The photos at the bottom help show the process.
What you'll need:
2. While paper is soaking, apply a solid line of glue all around one side of stretcher bars and spread it a little bit - see photo. Also note that in the photo, my stretchers have layers of paper along the edges from each time I've used them. I've used the same stretcher bars for decades.
3. Pull paper out of bathtub and let drip until no more water drips off the sheet. It's important to completely let the water drip off; any residual water might result in an annoying watermark in the middle that can't really be painted over.
4. Lay sheet over prepared stretcher bars so that it lines up evenly on all edges. Then press down on all edges so that paper is affixed to bars. Tug a little on all edges so that paper doesn't sag in the middle. (It's ok if there's a little sagging; the paper will dry up to a flat surface as long as most of the edges are adhered well to where the glue is on the stretcher bars).
5. Let the contraption dry for at least 12 hours. Putting it on the top of a clothes drying rack works very well.
The end result is a perfectly stretched, sized sheet of watercolor paper that you can paint directly on, while it's still on the bars. It's lighter and more portable than paper taped to a board. I usually do several paintings on one full sized sheet, then cut them out from the stretchers with an exacto knife when they're done. To reuse the stretchers, just pull off any excess paper that isn't glued. It's ok if there's still glued paper along the edges; you can re-use the stretcher bars for this purpose over and over. I've had mine for decades, there's a solid build up of paper on the edges, but they still work wonderfully.