I have been painting on watercolor canvas for about a year now and these are some of the problems I encountered.
Because the paint lays on the canvas rather than be absorbed into it, it takes a lot of practice as well as care if you try to paint over certain areas.
For instance, if you try to shadow, the layer of paint beneath will come up. Therefore, I put a light wash of acrylic on first so the white of the canvas doesn’t show thru unless I want it to.
Also, you have to spray the finished painting with Blair matte damar varnish (about 4 or 5 times). That way, since the painting doesn’t require glass, if it gets wet later on down the road, the paint won’t come up.
This canvas takes a lot of getting used to, but with some of the textured things I do the effect can be quite nice.
Look at the photo. To get that texture I used Masa paper that is in the rice paper family.
When you get it, it will be a smooth piece of paper and do not appear to be rice paper at all. You put it in a sink of cold water, wad it up into a ball, this breaks the sizing of the paper. It is now VERY VERY fragile so be careful when you spread it out. Take a roller and roll over the paper, otherwise it will dry puffy. After the paper dries, I adhere it to the top half of the canvas using Acrylic Matt Medium (diluted).
I again let it dry and then with a large brush with a good bit of water, wet the top half of the canvas. Next step is to have a lot of paint on the brush, most of the time I have two colors on the brush, one color on one end and the second color of the other end.
When you put the paint down, and with all that water that you put down, it just explodes into all the creases that the paper now has. After a bit, when your frustration levels off, you will enjoy working with it.