I know this is a old post but the hand sanitizer stuff works. I had some old ones where the buttons were not stuck, they just felt a tad jammed. They would still come up and react but they felt odd, heavy a tad sticky. I used the sanitizer on 3. And it worked pretty good. But one started doing odd things. So 2 out of 3 worked, I think the other one broke. But it was a vary old controller so it may have been the battery? And I used a tun of it on each one. I would say just dont use that much and it works. And you may have to do it a few times and keep pressing the button (press the heck out of it) and it gets back the feel again.
I understand that this post is very old, but i like to keep my old consoles up and running. I tried the wd40 (worked for a couple of minutes). Opening up the controller is a PITA. Buying a decent new controller is unreasonably expensive for last gen. I got a free controller from a chronic Dorito's eater off Craigslist, worked fine for a couple of months. What ended up working for me was squeezing a couple of alcohol swabs into the openings around the powered off controller. The square button hasn't stuck for over a week now.