How to get 5 gallon buckets apart?

Hot water should make them pliable so you can get them apart. Or spray with WD 40.

Wd40 works great.

As you've probably figured out by now the reason the buckets are so hard to separate is the vacuum, or negative air pressure, that forms when you're trying to pull them apart.

By applying force trying to pull them apart you create more negative pressure making them even harder to pull apart.

So now that we understand why this is difficult... What can we do about it?

You could try to equalize the pressure by warming the buckets and the air inside, in theory this may work. As the air molecules heat up they begin to move faster and bounce against the buckets more reducing the negative pressure slightly. The down sides to this approach are obvious though, heating the buckets will also cause the plastic to expand which may make separating them more difficult and heating the buckets isn't always easy.

As mentioned you could try to lubricate the buckets as you pull them apart, once again this may work, and once again it has downsides. Even though the lubricant may reduce friction it makes the vacuum problem slightly worse because you're filling the space that could have allowed air to pass through.

After some experimentation I've found that rinsing the buckets with water and applying a little pressure and being patient may be the best method.More specifically:

1. Spray down the buckets inside and out with plain water, mostly to clean them, but it may provide some small amount of lubrication.
2. Wedge a flat head screwdriver between the top edge of one bucket and the rib of the next.
3. Gently turn the screwdriver and wait a second for the pressure to equalize.
4. Rotate the bucket a couple of inches and repeat steps 2 and 3

After going around the bucket, you should be able to slowly pull the buckets apart.

Once you have your buckets separated you can prevent them from sticking in the future by placing a piece of cardboard or folded paper between the walls of the buckets when you re-stack them.

Try filling the top bucket with HOT water. It will contract and become a tiny bit smaller that the one under it. Wiggle them apart. Do the same for the next one. This would always happen with five gallon pickle buckets where I worked many years ago. This method always worked for me. Good luck!

Pour vegetable oil between then and twist.

Thank you!