I've heard the terms "mediums" used over and over again when using either oils or acryllics, and I've seen several different varieties in the art store...what exactly are they and how/what are they used for?

Just one of those things where I need to know... :-)

the Artist's Gloss medium makes good water........

A "medium" is most commonly a form of art, or something similliar

Ex. #:-) 's (Bob) favorite medium is pastel and paper, while 's (pablo) favorite medium is water color.

I thought that a medium is a necessary item that is required to be able to apply a certain type of paint to a certain "subject" like paper or plastic. Thinner for example would be a good medium for enamel paints.
Waterbased paints need water as a medium to be able to use them on paper etc...
But I was actually just totally stunned by the beautiful eyes of the lady who explained to me.. so don't blame me too much if I'm wrong here guys....

Mediums are the term referred to what paint is made of.

Oil paint has three parts: pigment, vehicle and medium. The medium in oil paint is either safflower oil, linseed or some other oil like walnut for example. The vehicle would be turpentine or paint thinner. In acrylics the medium is polymer and the vehicle is water. Pigmentation is medium indifferent and you can mix your own paint from pure pigments using a vehicle and a medium of your choice.

You can purchase premixed mediums that have different qualities to aid in the painting process. In oils, mediums can be used to create transparency for glazing, impasto (thicken paint), speed or slow the drying time, etc. In acrylics there are similar types of mediums. Gel medium for example is a thick polymer emulsion that can be used for various effects including making water effects as it dries clear though looks milky when wet. It can come in matte or gloss.

so what would something like Gesso be? It's starting to become a little more clearer now... :-)

Gesso is a primer (also called a "ground" or "size") for preparing canvas or masonite to receive oil, acrylic or perhaps other types of paint.

Gesso has been around for a long time and there are many recipes for its ingredients. However, one ingredient is calcium hydroxate (plaster) or similar material. This is what give it "tooth" for drawing the paint into the ground as opposed to floating on top. It serves to bind the pigment into the gesso providing a flexible adhesion. This is necessary as canvas tends to expand and contract due to environmental conditions and you do not want the paint to crack or flake off.

Silly question - has anyone used gesso as a primer say, on figures or their vehicles?

When I was at art collage...after the boer war...lol....
Mediums were the types of paint or pencil used to produce a piece of art.

Rough Guide to mediums used in art...

Non Paint Mediums.
-Graphite Pencil - Sketches/illustrations
-Charcoal- similar
-Coloured pencil - similar
-Pastels - similar

Paint Mediums
-Watercolours
Transparent pigments that are usually blended with water relying on the diluting action of the water to produce the desired effect.

-Gauache or Tempura (Poster paints)
Qpaque pigments thinned with water. Can use one colour on top of another unlike watercolours.

-Oils
Pigments needing a spirit based thinners or linseed oil. Can be applied thin with brushes or thick with palette knife.

-Acrylics
Can be used in a similar way to Watercolours/Guache or Oils, depending how thin the paint is used.

- Inks
Applied with brush or pens (separate nib type)
A modern equivalent of pen and ink is the graphic or technical pen used by draughtsmen e.g. Rotring.

Of course art works can use combinations of mediums
such as Pencil sketch coloured with watercolour or ink washes. And now theres rattle can art and airbrush art as seen on sexy custom cars, vans and trucks.

Anything else is a modern adoption of terms.

You DID ask...lol.

Hope it helps
Cheers
Peter

:-)

While that may be a definition of medium (or media, as in multimedia) I think the orginal question was specifically referring to these:

http://www.dickblick.com/categories/mediums/

As to using gesso as a primer for modeling I don't think it is a real good idea. The problem is with the granularity. Something like Floquil primer has microscopic granularity to the pigment which makes it ideal for scales we use. Gesso is too thick and since it is water based doesn't adhere real well to plastic. You can get gesso in a spray can and all I can say is try it for yourself. I have used the spray for figures but didn't like it compared to something like flat gray car spray primer or similar.

excellent explaination Bob.

Thanks for all of the replies guys, great clarification! As far as using gesso as a primer, I think I'll just stick my Krylon white primer... :-)