Why Gesso? And what is it.

I was asking this question not too long ago.

First off, Gesso is a primer that you put on a canvas. You use it either with acrylic or oil paint, but never with watercolor. It has a chalky quality when dry and can by layered to create a smooth working surface if you're working on a rough surface. Sanding between layers helps to create smoothness.

Different kinds of Gesso:
Gesso: This is just labeled "gesso" and it comes out white. This is your average gesso, and it is used for smoothness. Most are self-leveling so visable brush strokes will not be there once it has dried.
Clear Gesso: This has the same consistancy as the regular gesso however instead of it being white it will dry somewhat clear but has a foggy appearance.
Dark Colored Gesso: This also has the same consistancy as the regular gesso but it is black.
Super Heavy Gesso: This is white and is not self-leveling. It will stay in the place that you have shaped it once dried. This is made for texture.

You made need multiple layers of gesso before your surface is completely smooth, or to your liking. Make sure that each layer is dry before sanding or painting another layer (touch with the back of your fingers and if its cool to the touch it's still wet). You'll also need to wash your brush out with soap after each use with gesso or it will ruin your brush and become hard.

Now why use gesso?
I use gesso because it keeps my acrylic paints from soaking up into the paper. It keeps the colors more vibrant and from drying out on the paper too fast. Because I use watercolor paper a lot for painting, it would soak up the colors a lot faster; I also live in a very dry climate so I basically painted two strokes and the paint was already dry. But with the gesso I found the paint went much further and it didn't dry up as fast.