Gesso is an acrylic based and therefore waterproof when dry, but is great for creating very smooth surfaces on mdf boards card and watercolour paper. It can be applied with a brush, a palette knife if you want a little texture, or even with a roller.
If you use the traditional wet on wet techniques you have to be aware of puddling because the surface is not absorbent, but the effects you can create if you work quickly can be wonderful. Working wet on dry also requires a slight change in application. Because the paint is not absorbed, but sits on the surface, floating a wet layer of paint has to be done when the first layer is completely dry, and I mean completely dry. It also has to be done quickly and with one stroke. Sweeping the brush backwards and forwards will cause the two layers of paint to mix.
The great advantage of using a gesso ground is that paint can be removed easily with clean water and a cloth without staining. So it is possible with a fine brush and clear water to remove fine lines of paint revealing the white gesso ground, which is great for fine detail.
It is well worth having go with a gesso ground, but be prepared to adapt your technique to a none absorbent surface. Finished works do need to be displayed under class to stop damage to the surface.