Simple question. If a car’s door lock is getting difficult to open with a key, and assuming the reason is dirt or rust, is there a better product than WD40 to use to un-stick it?
First, I wouldn’t assume it is dirt or rust. It could just be a lack of lubrication.
Second, check with a locksmith. There are specialty lubricants made just for locks.
If you can’t find a specialty lubricant, I have used bicycle chain lubricant, dry silicon spray, and WD-40. They all seem to work, but the chain lubricant seems to last the longest.
Graphite oil is what is recommended for locks. You can probably pick it up at any locksmith shop.
Mostly there’s not much wrong with the part you put the key in. The resistance usually comes from the mechanism in the door that connects the key part to the lock itself. At least that’s how it was. Now they may be electric, and I’m wrong again.
But wizard is right - graphite lock lube goes in the key slot. Not WD-40 or oil. They are sticky and will make things worse over time.
If it’s an older (more than 10 years old) car, sometimes the problem in not the lock cylinder or linkage, but the key itself. It gets worn down with use and no longer lifts the pins in the cylinder sufficiently to unlock it. Of course, using a graphite lube is also very important to maintain proper operation.
Everyone has given very good advice.
I will only add a little and maybe some emphasis.
WD-40 should never be near a lock. WD-40 is a Water Displacement. It is a poor lube or cleaner. Try spaying some on an old glass. Do it a few times over a week and then come back a week later. It will be gooey/sticky. That is going to keep you lock from working.
You likely need to clean the lock. You can use almost any zero residue cleaner. Next you need to provide a lube that will not turn to a gummy mess. I suggest a graphite based lock lube.