Thanks Maureen,
I usually give them a clean with soap and water with my fiances cotton buds, or Q tips as she calls them, and if needed I'll clean the whites with an electric tooth brush. I think the Remove zit will work on the ones with pen marks underneath, but I'm not holding out much hope for the permanent markers one (I have a couple). It's the ones where the whites are yellowed or greyed that I want to try to rescue. I'll email Twin Pines to see what they say as they have stated that the Formula 9-1-1 will remove grease and smoke staines. I'm not sure if they mean surface stuff or ones that have penetrated the pvc more. I'll get some next month and let you know the results.