Close, etc. the barn door after the horse has escaped
Definition 1 of 1
Pro Tip 1/3Timing Matters
The phrase emphasizes the importance of taking action before it's too late. 
By the time they added more lifeboats to the ships, it was like closing the barn door after the horse has escaped.
Pro Tip 2/3Futile Efforts
Use when someone's efforts are pointless because the problem already happened. 
Putting on sunblock after you're sunburned is like closing the barn door after the horse has escaped.
Pro Tip 3/3Preventative Caution
It suggests that preventative measures are more effective than fixes afterwards. 
They now check the equipment rigorously to avoid closing the barn door after the horse has escaped.