Aluminum panel has become a very popular surface to paint on in recent years. It's rigid, light weight (cheaper shipping!) and totally archival.
Although I'm now a watercolorist, I used to prefer wood panels when I painted with oils and acrylic because I liked the smooth surface for fine detail work. But over time, wood contracts and expands with changes in temperature and humidity. Stretched canvas is even worse. I've never seen it happen personally, but when I think about portraits potentially being passed down for generations, making sure my work is archival is important.
Every time I'm in a museum with a friend or family member, they ask some variation of this question: "Why do all the paintings have cracks in them?" Canvas swells and contracts at different rates than the layers of paint on top, which causes tension and stress. There are other reasons too, but that's a big one.
And it doesn't happen with aluminum. George O'Hanlon at Natural Pigments has done a lot of research and writing around the archival qualities of artists' materials and you can easily burn a few hours reading up on it.
So, when I decided to mounting some recent watercolor paintings, my first choice was to try aluminum. I have the metal cut to size, slightly smaller than my sheet of paper. Then I use a matte medium as an adhesive, spreading it on the aluminum and the back of the paper. These are sandwiched together, carefully avoiding bubbles or air pockets.
Then I stack a bunch of heavy books on top and let it dry overnight. When I come back it's perfectly flat and evenly supported. After trimming the paper from the edges, the painting is ready to be coated. Varnishing is not going to work if the painting buckles or bends, so mounting the paper to a good surface is an important first step.
I'll have more on the varnishing process soon! (here)