116:2 Utensils of wood, metal, stone and bone can be kashered by immersing them in boiling water (this process is called “hagalah”). If the item would be damaged by boiling water, such as if it is made of parts that are glued together, even if it is only the handle that is glued on, then immersing the utensil in boiling water is ineffective because of the concern that it may not be done properly. (This is also true if the handle is made of horn, which would be ruined by the boiling water – Mishnah Brurah 451:23.)
116:3 Before kashering a utensil in boiling water, it must be thoroughly cleaned of rust and such. It must be completely clean but stains make no difference. If there are dents in the utensil, they must be cleaned thoroughly and, if it is a metal utensil, one should put hot coals on the dents to make that area glow (libun), after which one immerses the utensil. (If one neglected to heat those spots to glowing before immersion in the boiling water, he should do it after – MB 451:25.) If it is not possible to clean the dents and cracks thoroughly, or if it is impossible to cause those areas to glow, then the utensil cannot be kashered. Therefore, knives with handles must be checked very carefully to determine whether or not they can be kashered in this manner. It is best, if one can afford it, to purchase new knives for Pesach.