Because the refrigerator and kitchen cabinets are used with cold food, the only concern is that some ḥametz crumbs might remain there. Therefore, cleaning them is what kashers them. Hard to reach places where ḥametz crumbs may remain must be cleaned with soapy water to befoul crumbs that may be there and render them unfit for a dog’s consumption.
When kitchen cupboards are made of natural wood, it is hard to clean their cracks and crevices from ḥametz. If one does not wish to wet them with soapy water, he should clean them as well as he can and cover them with paper or cloth (MB 451:115).