Repair membrane keyboard silicone dome

I had a similar problem, with the torn dome getting stuck down.

I first tried vulcanizing rubber from a bicycle patch kit. While it made a nice layer of rubber over the tear and could be strengthened by making multiple layers, it didn't stick to silicone well enough. It would start slowly coming apart from the silicone on each press on the dome.

Some generic glue behaved similarly. I'm guessing the approach of building a layer on top of the tear and the silicone is bad. Just gluing the tear back together would be better, but since the thickness is under 0.5mm there's not a lot for the glue to stick on.

What did work was cutting the torn dome top off and transplanting the same kind of dome under it. I left a bit of the sides of the original dome and didn't use glue or anything-- once it's installed, it stays there just fine. The result is a key that's a bit harder to press than the others but works just fine. Of course one key is missing because I needed the spare dome.

EDIT: I blogged about this with photos: https://hannuhartikainen.fi/blog/keyboard-dome-repair/

I'd recommend rubber cement. It will adhere to the membrane well and provides the flexibility needed for a keyboard. Get the membrane positioned as well as you can and put a nice clean layer of rubber cement on. I won't guarantee that it will feel exactly the same as every other key, but it should last you a little while until you get a new keyboard.