One of the things I don't like about painting on canvas is how the paint skips over the fabric texture and leaves white "pinholes". Acrylics are stiff and stick to themselves so pinholing is even more of a problem with acrylics than with oil. I was working on a background today and got annoyed with the holes and dug around and found a solution--a bit of cut-up sponge.
The sponge carries more paint than the brush does and you can really cram it against the canvas, filling in stray holes. Here's a patch where I went over it with the sponge:
The sponge can add some fun rocky textures, too :D