How can I remove pencil marks from a Vintage Poster without damaging the poster itself? I have an Original, 1939 New York World's Fair poster, with a pencil mark on it. I'd like to find out how to get the mark off, without harming the poster's artwork.
This "answer" is in response to HOW to remove pencil marks on paper. It is information based on my personal knowledge and experience as an illustrator, graphic designer, and part-time art instructor.
Important: This answer is NOT in response to how to handle valuable documents or collectable vintage items as noted in the comment to your question by Robert Cartaino. It is a good idea to consult a professional restorer. It might even be better to leave such documents "as is." This answer is not a recommendation to follow for valuable materials.
First: Pencil leads are composed of clay and graphite mixed in different proportions to be practical for their intended use. High clay content resists wear while low clay proportion leaves a darker mark for the pressure used. Leads are graded with a number or a letter H (4) for Hard, B (1) for black, and HB (2½) for the best of both qualities.
Second: Poster printing stock is a complex material of varying amounts of fibre and starch and with different surface textures which affect the paper's "tooth." Tooth refers to the abrasiveness of the paper surface and it's ability to hold a (pencil) mark. There can be a baryta coating on the surface of the kind of paper used for printing the poster. Paper cannot be repaired. Once fibres are broken (by folding or denting), removed or lifted (by scraping), torn, or warped (by moisture) the damage is permanent and cannot be reversed.
Third: There are many different kinds of erasers and techniques for removing marks on paper. Erasers can be hard or soft, stiff or flexible, natural or artificial, solid or semi-liquid. Some can be impregnated with various abrasives (pumice or fibre glass) to scrape or abrade the mark (and the surface). There are simply too many different kinds of materials to review in this venue.
Fourth: After identifying the type of eraser for the kind of pencil mark, the manner of erasing is to avoid working the mark into the paper base. Likewise, avoid damaging the delicate surface of the original document with excessive pressure.
How: Work under the best non-glaring illumination you can find. Clean the work area. Wash your hands. Use a CLEAN eraser. Clean the eraser frequently to avoid re-contaminating the marked area. Work across the mark from one clean side of the piece, across the mark to a clean area. Work a bit at a time working from one end of the mark to the other end of the mark. Some choose to mask (cover) the adjacent clean area with an erasing shield to isolate mark while you work on it.