Sanding under hood on front fender

I am finishing up my first fender and i need to sand where the fender goes down and bolts on just under the hood. Was wondering what is the easiest way to get at it. The palm sander or oscillating sander is to large and i really was hoping not to have to sand by hand.

Now im not meaning where i have the filler but the two step lip just above that. Its the only thing left on that fendr before i prime it.
Thanks Rod