Acrylic medium is the same formulation as acrylic paint, minus the pigment. It can be used as a paint additive, an adhesive, and a texturizer. It also can be used to create image transfers. It comes in three basic weights (medium, gel, and paste) and two finishes (matte and gloss). The different weights and finishes are inter-mixable, so you if you cannot find exactly what you need, or you are on a budget, you can create it using two or more products.
I prefer Golden acrylic mediums. They have a huge amount of detailed information on their various products available on their website (Golden Paints). They aren’t terribly expensive, especially if you can buy in bulk when it’s on sale. I buy medium by the gallon and pour it into smaller containers. If the lids are sealed tightly, it will last indefinitely.
Whether to use gloss or matte finish is largely a matter of personal preference. There are differences in how the two finishes handle, though. When wet, gloss tends to be “grabbier” than matte, so it is often preferred as an adhesive. It also tends to remain tackier after it’s dried, so it can be tricky to use in journals and books. This is especially true in humid climates. To alleviate tackiness, allow the medium to dry thoroughly, then either buff with waxed paper or dust with corn starch. It is also a good idea to place an interleaving sheet of waxed paper between book or journal pages, if there is any chance of them sticking together.
Matte medium includes silicates, to reduce shine. When multiple coats of medium are applied, those matting agents will build up, creating a cloudy appearance. To counteract this, use gloss medium as the next-to-last coat, then matte varnish on top of that. Or, you can use gloss medium for all but the last coat.
A third finish option may also be available from some manufacturers: semi-gloss. If you want a semi-gloss or satin finish, but do not have it available, you can create your own by mixing matte and gloss mediums.
Caveat: It seems to be commonly recommended in some groups to use acrylic medium as a finish coat, in lieu of varnish. This seems to be popular advice in the altered book and art journal communities. I think that’s bad advice. Because of its molecular structure, acrylic polymer is not completely impervious. Things like unstable ink, moisture, etc. can wick through the acrylic barrier. I’ve had inks migrate from the front surface, through several interleaving layers of acrylic medium and collage materials, and through the support, finally ending up on the back side of the support.
Acrylic medium is also not reversible, so if the surface is damaged, there is no good way to repair it.
If an acrylic finish is desired, my recommendation would be to use a layer of acrylic medium, then a layer of acrylic varnish. Acrylic varnish provides a protective coat, and if it is damaged, it is easily removed with ammonia.
Medium is the thinnest of the acrylic mediums. It is easily pourable, with a consistency like that of cream. It works well as an adhesive and for creating image transfers. It can also be added directly to acrylic paints, to extend them or to alter the consistency, transparency, and finish.
Gel is thicker than medium. It is pourable, with a consistency like that of thin yogurt or kefir. It works well as an adhesive and for creating image transfers. Gel can also be added directly to paint to alter consistency and transparency. It can also be applied to the surface prior to painting, to create texture.
Paste is the thickest of the acrylic mediums. It is too thick to pour, with a consistency like pudding. Depending on the manufacturer, it may be called either molding paste or modeling paste. While mediums and gels dry clear, paste may contain fillers that make it opaque. Paste is a good way to build up surface texture, prior to painting. It can also be mixed with acrylic paints, though be aware that its opacity will alter the look of the paint.