A post doesn’t have to be damaged to wiggle from side-to-side or lean in one direction. If it’s not damaged by rot, the fence was improperly put in or the post worked its way loose during winter freezes and thaws. A concrete girdle can stabilize posts forever — if they don’t rot.
One way to stabilize a post is to put a splint on it. You need scrap 2-x-4s, a .60 preservative that also protects against water damage, a hammer, and bolts. Here’s what to do:
Soak the 2-x-4 splints in preservative for a minimum of 12 hours.
Dig around the post so that you have room to fit a 2-x-4 splint beside it on two sides.
Slope the edges of the splints so water runs off.
Put preservative on the edges.
Pound the splints into the ground.
Secure the splints to the pole with bolts.
Use dirt, pebbles, or gravel to refill the hole.