rubs out

You know, a draftsman always makes his drawing in pencil first, and when it's done he rubs out the pencil lines with stale bread crumbs.

昨天他终于完成了用墨水描线。您知道，一个制图员往往先用铅笔打底，绘图完成后再用干面包屑擦去铅笔印。