Hi- Welcome to Just Answer and thanks for your question.
You have a few options:
1. Non-colored block erasers Erasers such as the noncolored Eberhard-Faber Magic Rub Eraser and the Staedtler Mars Plastic may still be useful for work on small areas.
2. Groom / Stick® is a natural rubber eraser that is a an excellent surface
dry-cleaner. A kneaded gum eraser may also be of help. Just remember to
be gentle with any erasing you do. Use short, wandering strokes to avoid
leaving erasure marks on the paper. Check as you work to insure that the surface
of the paper as is not being abraded.
3. Grated/granulated vinyl erasers can be used for cleaning most printed surfaces. Containers of granules can be sprinkled or rubbed across the soiled surface of the paper. After gently rubbing the granules into the paper with a circular motion, the granules will pick up the dirt rapidly. Then you can carefully brush the granules away.
4. Vulcanized rubber dry-cleaning sponges are good tools to use, provided you are careful with handling. See http://apps.webcreate.com/ecom/catalog/product_specific.cfm?ClientID=15&ProductID=2127Rub gently and randomly with the sponge to avoid creating a pattern on the paper surface. Finish by using a large soft brush to remove any pieces of the sponge.
Make sure you have a workspace with a clean, large smooth surface where your poster can be kept away from further damage. Begin the cleaning using the soft brush to remove loose dirt and dust. Be careful to avoid enlarging tears by brushing in the direction of the tear. After every few strokes, wipe the brush across a blotting paper or a clean piece of fabric to dislodge accumulated dirt and dust.
Be careful; aggressive efforts to remove dirt may cause you to inadvertently make a hole in the paper.
If you get tired of DIY, the people at Chameleon Restoration have experience with a wide variety of techniques for removing stains and writing on paper and cardboard. You can see examples of their work at http://www.chameleonrestoration.com/before2.html
Their prices are pretty reasonable (e.g. mostly less than $50. See: http://www.chameleonrestoration.com/prices2.html#posters for further price details. For more information, see http://www.chameleonrestoration.com/faq.html.
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