Using Mylar sleeves to protect and display posters is called encapsulation , a procedure used by people who work in libraries with the title paper conservator. These people would be horrified by the idea of "linen-backing" an important piece of paper, and if you have the time they'll spend the whole day telling you why it is a terrible idea. When you encapsulate a poster for display and preservation, you usually use a special solvent to deacidify the paper while you're at it. One of several companies specializing in archival products is univeristyproducts.com, but there are others too.