Great Planes 4oz bottle of milled fiberglass that is used to strengthen epoxy bonds. It is mixed with epoxy glue at a ratio of 1/4 to 1/2 teaspoon per ounce of epoxy. (ratio verified w/product manager KB 4/00). Increases the strength of any epoxy. Perfect for adding strength to firewalls, wing hold-down blocks, landing gear blocks. Easy to use. Can be smoothed over with isopropyl alcohol for a sleek finish.