Why a lock gets stiff, and why oil is a trap
A pin-tumbler lock is five or six tiny spring-loaded brass pins. Your key's profile lifts each one to exactly the same height — the shear line — and only then will the cylinder turn. A stiff lock means the pins are not rising cleanly. Three things do that:
- Dust and lint — pocket fibres, plaster dust and grit pack into the pin chambers and drag on the brass.
- Oil turned to sludge — someone sprayed oil in years ago. Oil leaves a film, the film catches dust, and the dust becomes glue. This is why a lock gets worse a few weeks after being "lubricated".
- A worn key — keys are copied from copies. By the third generation the heights are slightly wrong and every turn grinds the pins down.
And one thing that isn't the lock at all: if the key turns freely with the door open but fights you once it's shut, the cylinder is fine — the door has shifted and the latch bolt is now hitting the edge of the strike plate instead of sliding into it.
The one test that separates them
Open the door and work the key. With the door open the latch doesn't have to reach the strike plate, so every frame problem is excluded. Stiff with the door open = the cylinder. Smooth open, stiff shut = alignment. Two completely different repairs, five seconds to tell apart — and almost nobody does it before reaching for a spray can.
Why WD-40, 3-in-1 and thin oils are the wrong answer
None of them are long-term lubricants. WD-40 is a water displacer, 3-in-1 is a light oil: both leave a wet film inside the cylinder, and that film collects dust into gummy sludge that binds the pins tighter than before. Every locksmith source agrees on this, and the failure arrives two to six weeks later, so people never connect it to the spray.
- Powdered graphite — right for older pin-tumbler locks and traditional deadbolts. Lasts 12–18 months. Only downside: it's black and it migrates.
- Dry PTFE (Teflon) spray — right for modern cylinders, deadbolts and smart locks. Lasts 8–12 months, no stains.
- Silicone spray — for the latch bolt, hinges and strike plate, not the keyway.
One short puff into the keyway, then insert and withdraw the key a dozen times so the profile carries the powder down to the deepest pins. Less is more — you are dusting, not greasing. No graphite? Rub a soft pencil's lead over the key's teeth: it's graphite, and it will get you in tonight.