recommended tools for grinding back where you can't fit a grinder?

[pedrobedro]

A Stanley blade through the joint and then tearing along the cloth gives you a nice parallel belt.

You can cut them with a pair of scissors quite easily. I have an old pair of shed scissors "cos I'm not allowed her good dressmaking shears" and it doesn't seem to do them any harm cutting abrasive paper it even seems to make them sharper.