Someone I know puts a couple of very thin plastic sheets (cut up vegetable bags from the supermarket)under the tin lids of tins of varnishes, paints and glues that come stored in tins.
It makes the lids come on and off very easily and he claims they act like a 'washer' and hence give a more *airtight* seal. But since air passes through plastic bags (why long life bread is packaged in 'silvered' plastic bags) albeit slowly, would this be worth doing?