wvnimrod

A freind just gave me three blocks of styrofoam to use as targets, he gets them from where he works they are 4'X3' and 2'thick
I have shot about a hundred arrows at one of them and they stop arrows really well,but I noticed that some residue builds up on my arrows after a few shots.
Has any of you used styrofoam targets before and had this residue problem? And if so whats the best way to remove the buildup from your arrows?
I am hoping to find an easy solution to the build up,due to the fact that my freind can get these blocks at anytime,so I will never have to worry about targets agian.
Maybe I can even carve 3D animals out of them with the hot knife he gave me!