Is there any relatively simple way to sharpen images?

[denzjos]

I often use arrows on technical sketches using tranparent layers. Sometimes I don't like the sharpness of the arrow and try to manipulate the arrow with curves and levels. It seems to have no effect on the trasparent layer. See attachment file. On the transparent layer curves and levels have no effect. On the white layer curves and levels have effect.

  transp layer curves etc.xcf (Size: 14.44 KB / Downloads: 131)
Ps : I know a few other sollutions to make sharp arrows, but in this case it is concerning curves and levels on a transparent layer.
akovia, if my workflow is wrong, please correct. Thanks in advance.