Hi Paul,
I allways use a fibre-pen, this is a pen filled with a glassfibre cartridge, which you can roll out on the backside. You can buy this gadget in an art store, 'cos it's the artists who normally use the, for re-touching works.
It works like a gem, 'cos the fibre is flexible, be advised though, not to inhale the fibres which came off, and be sure not to touch the fibres with your fingers, because it's very thin glass, and if they end up in your fingertip, it'll take two or three weeks to get them out!!!
HTH, John.