Never ever use an e-file on the natural nail! All you need to do is remove the shine not put hundred tram lines in the nail!Now im sure someone will correct me if im wrong as i have limited knowledge of acrylic and no knowledge of e files.
I thought that if a technicians application and products were up to scratch there is little need to use an e file.Good products only need the nail to be shine free to adhere. They also file down easily and you should be able to do most of your refining with your brush so an efile is unnecessary.
This is only my opinion.