Because they are used with cold food, the only concern is that some ĥametz crumbs might remain there. Therefore, cleaning them is what koshers them. In hard to reach places where ĥametz crumbs may have gotten stuck, one must pour soapy water or some other substance that will befoul the crumbs and render them unfit for animal consumption.
When kitchen cupboards were made of natural wood, they often had cracks that were difficult to clean completely from ĥametz that got stuck there. Aĥaronim therefore ruled that the shelves should be covered with paper or cloth (MB 451:115). However, there is no concern that ĥametz remained in smooth shelves like those used today. Therefore, once they have been cleaned properly, they need not be covered with paper or cloth.