I have Tinkering III, but every time I try to repair it, it only goes up to 99%. Because of this I cannot complete "Tinker with your Tools" quest. How do I repair it?
Using a Tinkertool to repair a tool (even itself) causes wear to it. When repairing itself, this wear is applied after the repair, effectively capping the tool's wear at 99%.
The only way to repair a Tinkertool to 100% (say, for the quest) is to drag it onto a second Tinkertool.