When I want to go back and add some light areas to a painting, I lift out places with a thirsty brush if the paint is wet, but if the paint has dried, I use an old typewriter eraser (round rubber with a little brush). The erasers have a gritty quality that tears into the paper, so use them gingerly. Of course typewriters, as well as the erasers, are fast disappearing. I tell my students to buy those erasers before they disappear.
If the paper becomes too rough, especially if you need to add more paint, rub the paper down with the back of a spoon. I keep a plastic one next to me while I paint. Never use a silver spoon; it’ll mark the paper.
Ross Merrill is chief of conservation at the National Gallery of Art in Washington, D.C.