I have been using this product since year one & have found two pretty good ways to test it & care for it .Try to keep the packages sealed if possible. When you open it ,smell it ,if it has an oder it still pretty fresh. When you pull the backing off ,grab a corner & shake it, if it sounds like wrapping paper it is getting old. It will still work OK but a lot of life is gone from it & after a while it will get very brittle on your open bay surfaces. What we need is a great big "baggie" to store it in! Use my old stuff on sheeted surfaces & buy new for the rest. Try to keep your plane in the shade helps a lot.>>>>>>>>>big max 1935>>>>>>>>>>>>>>