If I make a drawing on a layer and it is surrounded by alpha and choose to make a heavy transformation the line crispness is lost because the edge becomes blurry.
This gives me 2 ways out to fix the blurry edges:
redraw the lines
erase the edge
Both options are time consuming.
Is there a way to setup a sharpen filter just to the edges of the drawing quickly or some other solution for this blurry edges drawing without total manual reconstruction?
You have a couple options. As @Deif_Lou said, I personally use Filter -> Adjust -> Color adjustment curves and set the channel to alpha, and adjust it until it crisps up the line.