Here are some tips to keep in mind when storing paint in your garage. Once you’ve finished your painting project, clean the interior lip of the can and make sure that there is no wet or dry paint left in the channel. Do the same for the lip of the paint cover. It will give you a secure seal when you close the paint tin. Use a rubber mallet to avoid damaging the paint tin.
Always store paint in an area where it does not get too hot or cold. Putting it in a tin shed is not advisable. Extreme temperatures affect stored paint in different ways. When paint gets very hot, it dries up. When it gets too cold it separates and forms into a curdled mess. Heavy moisture can rust paint cans, and if any of that rust gets into the paint, it will ruin it.
One of the best ways to seal your can is to wrap it in glad wrap and place a rubber band around the body near the top of the can. I advise using a proper paint can opener rather than a flat head screwdriver that can damage the lip of the can.
Putting your paint can in a sealed plastic storage box is about as safe as it can get. It has an extra layer of protection from the elements. It’s protected from moisture and humidity. It insulates the can from extreme temperatures. If you ever need to do a touch-up, you'll be confident in the knowledge that the paint you stored will still be good. - EricL
An article about painting recommends that if opened, "clean the rim and place a piece of plastic wrap over the top, then hammer on the lid." Does anyone do that? I just tap the lid back on and the paint seems fine. The only problem I've ever had is with a really old tin that has rusted and contaminated the paint. - AHoy
I have done the glad wrap thing. Seems to just give the lid a better seal. With regard to how long it will last depends on how well the lid is sealed. Most manufacturers only recommend storing for up to 2 years. I repainted one wall at my mum's place last year and then found out the paint in question was close to 10 years old. Apparently nothing wrong with it. I might add it did seem to take longer to stir the mix even again. - darylhewston