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Sometimes I fish around in my shed for a repair kit and pull out a pair of patches which have been sat around for a while (a year?) I find the patches are stiff (even though they're still in their foil).

I proceed with the repair but suspect it isn't as sound as it would have been with a freshly purchased malleable patch.

Tonight, while doing a repair, I actually looked at the patch and the kit box to see if there is a Best Before date (there isn't).

But what is the useful shelf life of an unused patch?

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I generally toss my patch kit every year or two. Sooner if the glue has been opened, later if not. I would guess that an unopened kit is good for 3-5 years, and the main clue it's getting old would be that the glue is too stiff. (I basically don't get punctures since switching to belted tires.) –  Daniel R Hicks May 11 '12 at 10:48
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It depends very much on how the patches are stored. I have a box of strips of patches that lasted about 10 years before the patches became unusable. But that was a whole box, in a plastic bag, in a cold dark place (a box of bike stuff in the garage). On the shop counter that same box would probably age out within 3-12 months depending on how often it got direct sunlight. What matters is heat, how well sealed the container is, and light (probably in that order). I think you're doing the right thing by inspecting the patch and deciding whether to use it.

That's not too useful if you're really asking "how often should I buy a new patch kit". In my experience at least two years, normally more like five. Again, I store my patches in a patch kit (plastic box), in a plastic bag with some tools, in my pannier. It's pretty cold and dark in there too. I find that the glue tube gets really gummy and useless before the patches wear out. But since I wear my tyres pretty much down to the canvas I go through a few patches most years, so that's not normally an issue. (I run the tyres until I get a puncture that seems to be attributable to tyre wear).

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