Main Lining Boston. Boston’s underground infrastructure is old and deteriorated, making main lining an important piece of the rehabilitation. Perma-Liner receives calls on a weekly basis from Boston contractors about our main lining materials. The Perma-Main lining material is multiple layers of felt and one layer of coated polypropylene plastic. The coated layer is sealed at the seam and felt production is achieved by a non-woven process. If you live in Boston and are interested in becoming a Certified Perma-Main Lining Installer, you know who to call!