I just responded to a question about what type of adhesive I use when collaging, and I thought I’d share my response here because I get this question a lot.
My favorite adhesive for collage is Liquitex Acrylic matte gel medium. I like the matte because it does not get sticky like the gloss and is more workable with other media, like pencil for example. I find that it is also better for layering than the gloss. I also prefer the matte over gloss because the matte does not reflect light, which to me can interfere with the work (you can get glare with the gloss.) When I am doing assemblage, I use heavier glues like a hot glue gun, or Weldbond. There is also a paste called Yes! that works well with acrylic and is great for heavier papers and materials. It can also be thinned with water, which makes it very flexible.
I use several types of burnishers to smooth and flatten elements, and to get rid of air bubbles. One of my favorite tools is a roller used for block printing. It really depends on the material being put down though, as some papers are delicate and require careful application. Ultimately it comes down to what looks and feels right while you’re working, as well as trial and error.
I seal most of my collages with a matte acrylic varnish. Again, I prefer the matte because gloss gets glare (matte is easier to scan and photograph too.) Sometimes a gloss is appropriate, but I prefer the matte most of the time. I’m partial to Liquitex because I’ve been using it for 20 years and I’m familiar with their products, but I’m sure brands like Golden or Dick Blick work just as well. It really comes down to preference I think.
So there ya go!