Dave Rowe wrote an article in Model Railways about 25 years ago, where he pointed out that each time you open, stir and use the paint, bits of paint will remain on the lip, and be transferred to the lid, which gradually builds up a crust. He was assiduous in removing any paint from the lid before putting it back on - forcing it on when the crust is there merely deforms the tin and lets more air in. He also used to put a drop of thinners (brand matched, for this purpose) into the tin before closing up, and then stored them upside down so that the paint would naturally seal the joint - and also, incidentally, collect all the pigment where it was easier to get to for mixing next time.
Before anyone comments that they don't have the time/patience for this, Dave was (is?) famously impatient* and by his wife's admission if not his own, tight with money. He saved the latter by not having to buy so many new tins, and kept his patience better, too, by not opening a tin to find it has dried up.
*He reckoned that if you measured up correctly, checked, and took time to make several light cuts, it would be quicker than taking a short-cut and making a mess of everything, only to have to start again and do it properly by measuring carefully, checking, and taking several light cuts... He must have had some kind of satori, as he reckoned that the truly impatient person does not hurry things. (Wonderful paradox, that!)
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