It is one of the cheapest supports that exist, but not for that reason of worse quality. The gray cardboard also called cardboard stones for its hardness, is made with compact recycled paper and glue. It's very tough.
For painting it can be used both prepared and unprepared. If you use it unprepared, you should apply a 50% water and glue on the back to avoid surface tensions and that it does not arch or ondulate when painting. In case of not making a previous preparation, it will work well for acrylic and oil, but it must be taken into account that it will absorb a large amount of paint during the first coat.
The second of the supports for painting is the best known as well as the most vulnerable. Paper is a thin sheet made of vegetable fiber pulp or other materials ground and mixed with water, then dried and hardened.
On paper you can work with all the techniques, using the most appropiate for each of them. There are many types of papers for watercolor,
although it is common to use them of a greater weight despite having a somewhat higher price. On the papers you can also paint with acrylic paint and oil, although there are specific papers for these painting techniques.
The canvas is a fabric of linen, cotton or hemp, which serves as a support for the pictorial arts. In other words, the canvas is a fabric itself, although this is usually the name given to the set of fabric and frame.
On the canvas you can paint with any technique although for this it will have to be prepared in one way or another, thus varying the absorbance of the canvas. These preparations can be synthetic with fillers os traditional based on rabbit tail.