@konrad911 ,
The best tool to use is a Ā¼" driven electric or air handheld grinder with the appropriate shaped rasp (cutting head). It is good to have an assortment of different shaped rasps and cutting heads. An electric Dremel could be used but the time required and cost of materials would no dought be higher.
Then finish using drum sanding with the same tool; I use 120 grit then 240 grit and DONE! I do have my machinist knife edge, polish and then perform a complete bottom end balance.
You can also use a cut off wheel and feather and fare out the areas you chose.
I remove every mark, marking, casting, all <90Ā°'s getting a chamfer and all edges >90Ā° get rounded. I removed all roughness but did not go for a mirror, just fared, smoothed.
While i haven't had the need to peen the my cranks, having them peened would be the appropriate next step. After I perform this process on my connecting rods, they are off to get peened (plus ARP hardware).
I prep the block and main caps and to a much lesser degree the pistons. The only component I do not do this on is the cam.
Two other components that benefit from this treatment are the water and oil pumps.