Methods for hand washing a poster depends on the type of stain. Some difficult stains should only be treated by a professional restorer who will have knowledge of the correct chemicals to use without damaging the poster, notes Learn About Movie Posters.
If a poster is stained with mildew, it may be possible to remove the stain with hydrogen peroxide, according to Good Housekeeping. Mix one teaspoon of 3 percent hydrogen peroxide with one teaspoon of distilled water and apply it gently to the mildewed area with a cotton swab.
To remove tape from posters, try heating the affected area and peeling the tape back. If this doesn't work, apply Goo Gone and remove the tape with gentle strokes, states How To Clean Stuff.net.