French chalk/soapstone/unscented talcum powder, all the same thing really, can be bought from Boots, or Cycle shops, and can usually be found in cycle puncture repair kits, in stick form, basically, it stops the rubber sticking to itself. On a windscreen rubber it will allow the screen to seat properly, and reduce the likelihood of the rubber tearing, while pulling the string out....
But thinking about it, if you get a new rubber, it'll probably already be coated anyway, so clean it off after fitting, and not befors...
Thom
