The story is "*The eternal enemy*". You can read a recap [here][1]. 

[![Book Cover - The External Enemy][2]][2]

The main storyline follows Rela, a teenage girl who acquires a state-of-the-art VCR that, much to her amazement, shows events in the future. She then proceeds to attempt to avert fate repeatedly. After averting her own death, and spotting a creepy man in a jogging suit with now seams, the twist you remember is revealed.

> Creepy dude comes in and takes the bandage off Rela's hand from where she cut it at the party. He then PEELS THE SKIN right off with it. All the skin from her hand. Yet, she feels no pain. RELA is Robotic Experimentation Logistical Algorhythm. The guy is Grandfather. He's gonna destroy Rela with a ray gun, but first...

> Remember Sara? Born in 2036 in LA, in the aftermath of a world war and during an American depression. Sara's grandfather disappeared when she was 10, and reappeared when she was eighteen. He told her about some stuff he'd been working on in the meantime... i.e. making human cyborgs that can travel to the past. You know, everyday stuff.


  [1]: http://likepike.blogspot.se/2008/09/eternal-enemy.html
  [2]: https://i.sstatic.net/PNlrZm.jpg