Things You'll Need
You did everything right--you carefully sealed the can of paint after you were done with the job. You labeled it and put it away safely in case it was needed for touch-ups later. Now you need to touch up the wall, but the darn paint lid is rusted shut. It just takes a little common sense and a few professional tips to keep paint fresh and to keep the cans from rusting once they've been opened.
Avoid working straight out of the paint can. Instead, pour the paint you're going to use for brushing into a separate bucket and work out of that. This keeps the rim of the paint can clean, so it's easier to close properly when you're done.
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Rinse the paint can lid until all the paint is cleaned off. Using a damp rag, wipe around the edge of the can, removing excess paint.
Use a hammer to gently tap down the lid until it's tightly sealed. Don't whack it too hard--this isn't necessary and crimps the metal, allowing air to seep in and rusting the edge, making it more difficult to open later. Laying a block of wood or a rag over the lid before using the hammer will also prevent you from crimping the metal lid.
Choose a cool, dry place to store the paint can. If possible, use a shelf rather than a damp garage or basement floor. Humidity and moisture cause rust, so the drier the storage area, the better. Store the paint can upside down. This sounds counterintuitive, but it works very well. The wet paint in the can will automatically seal the opening, keeping the paint fresh for years.
Wipe a very thin layer of petroleum jelly around the rim of the can before sealing it if you live in a very humid area. This will help keep the lid from rusting shut.
If the paint is being stored for a long time, check it about once a year. If the can has started to rust, it can be poured into a new can.
Make sure any stored paint is protected from freezing. Date and label paint cans for storage. You may think you'll remember which paint was used where 3 years from now, but most people don't.
Avoid leaving paint near any heat source.