How to protect aluminum fence posts from weed trimmer?
Get a piece of approprietly sized PVC pipe or corragated plastic drain pipe.For a 4 inch post use pipe with at least a 4 inch inside diameter Cut it into 6-9 inchs long pieces. Slice the pieces so you have 1/2 of a piece of pipe. Bury the pipe about 3-6 inches into the ground  so it covers the base of the post. Fill the space between the pipe and the post with crushed stone.