Repainting a canvas tips

I have a few really ugly canvases that I got in a lot. I want to repaint them with a design I'll like and actually stitch. Any tips for either removing paint from a canvas or painting over? The current design is really dark. I tried to cover it in white but it's showing through. I'm nervous to put more coats of paint on the canvas. Any tips or suggestions? I thought about just turning it over but some of the dark paint is on the back too.