I like to recycle or up-cycle materials for my art, including my own stuff! That's one reason I like working with canvases and boards that have a cradled edge (at least an inch, preferably more). I'll paint something or make a collage, and I'll know whether I like it right away, usually. Most of the time if I don't like it I'll just leave it and figure maybe it will grow on me. But it never does. So I'll either add something (harder to subtract) or paint gesso or watercolor ground over it. Or if it's really dark, paste paper down on top of the painting. I love Daniel Smith Watercolor Ground. It can be pretty thick and has some texture (which is good for watercolor) and you can always sand it if it looks rougher than you like. You can use it on any material but if you use it on smooth metal I recommend sanding the metal first to scratch it up a bit. I made some small pieces from Altoid-type tins and through trial and error I figured out that sanding is the way to go.
Anyway, back to paint-overs (or collage-overs). Here are two pieces I recently finished. The bug idea emerged when I was wondering what to do with the sketch + paint version of the bug I painted on the floor. I liked it so I wanted to use it in some other way. So I grabbed one of my less-than-successful paintings and collaged and painted over it. I like doing this because then you don't have as much art hanging around that you aren't very enthusiastic about.
I did the same thing with the cat piece, which had been a collage I'd done quite a while ago. I liked some things about it but not others. I had used a photo of a bird in the original composition and I decided I didn't like including that kind of image in my work. So I started messing with it. The other thing I like about this process is that it frees me up. Since I'm working over something that I didn't particularly like, I'm not inhibited by the worry that I'm going to "ruin" something. It's just fun. So here they are.