+1 on posting a picture, but if I'm understanding you correctly, the tool you're asking about is a corner trowel.
This tool might not be necessary though, depending on the amount and severity of the dings you have. You're using a sanding block
, right? With a little compound directly on the corner and on each side, you should be able to shape a sharp corner with a block.
I use Dap DryDex for my repair work. It's excellent stuff - much easier and more convenient than having to mix, and it dries rock hard.