Fixing an old mechanical keyboard
My keyboard is about twelve years old. Over the last year the space bar started sticking, the E key needed two presses, and the left Shift gave up entirely. Instead of buying a new one, I spent a weekend on it.
Cleaning
All keycaps came off with a wire puller and went into a bowl of warm water with a bit of washing-up liquid. While they soaked, I went over the plate with a soft brush and a vacuum. The amount of dust and crumbs that came out was slightly embarrassing.
The keycaps need to be completely dry before they go back on. I left them on a towel overnight.
The sticky space bar
The space bar uses stabilisers — thin metal wires that keep long keys level. The old grease had dried out, so the wire was scraping against the plastic. A tiny amount of dielectric grease on the ends of the wire fixed it completely. It is the cheapest improvement you can make to an old board.
Dead switches
Testing with a multimeter showed that the E switch was worn and the Shift had a cracked solder joint, probably from years of being pressed hard. Re-flowing the joint fixed the Shift. For E, I desoldered the switch and replaced it with one taken from a key I never use (Scroll Lock, finally doing something useful).
Desoldering was the slowest part. A solder sucker works, but desoldering braid gave cleaner results on this board. Patience and a bit of fresh solder on the joint first make the old solder flow much better.
Result
Everything works again, the space bar is quieter than it has ever been, and the whole thing cost the price of a small tube of grease. The board will probably outlive the computer it is plugged into.