[Note: You may also enjoy reading More on using glues.]
Gel medium is the magic potion of the art world–it’s a glue or a sealer; it creates a matte or shiny surface, it thins acrylic paints and makes them more transparent, it slows drying.
And the vary characteristics that make it wonderful cause confusion. Let’s keep it simple: Gel medium is acrylic paint without pigment color.
There are three textures: thick, thin and medium. That’s where the first problem starts.
Thick is also called “heavy,” and “gel,”
medium is often called “fluid,” or “soft fluid, or “medium gel,”
liquid is also called “most fluid,” “self-leveling,” or, “liquid”
Confused yet? I use Golden, which is a brand name, not a color, and their viscosities are divided into Gel and Medium, which is fluid. Golden makes at least 20 variations, including gels packed with glass beads for shine and texture, and those you can shape into peaks for impasto.
I use heavy gel when I need a paste, a medium when a lighter glue is needed, and a thin for coating collage work, isolating elements that might bleed or deteriorate or have a high-acid content which interferes with the archival qualities.
In addition to their consistency, gels also have transparency. Matte is great for creating a see-through surface without a shine. But if you are going to build it up, it will reduce clarity. Satin has a soft luster, and gloss is perfectly clear. It is shiny, but you can also build up layers and get that trapped-in-lucite look that reminds me of decoupage gone wrong, but certainly has its place.
Almost all mediums extend drying time of paint, and your best bet is to use the thinnest consistency and apply several coats. Let dry thoroughly between coats. If you use a heat gun, be careful not to hold the nozzle too close to the piece while the medium hasn’t set. Your collage will absorb the liquid, form steam, then develop blisters that can pop, spraying hot medium.
If you press your pieces, make sure they are dry. Even then, I use parchment to separate pages when they go into the book press.