Yeah thanks to youtube! I was watching a video from one of my favorite Comic Painter, Grzegorz Rosiński, I could see he was peeling off that weird stuff, then found out what it was.

http://www.winsornewton.com/na/discover/tips-and-techniques/different-art-masking-fluids-us

He was using it to cover the gutter in comics pages which explained me finally how they paint with large moves without messing with the exisiting shapes or borders.