Cleaning the Utensil
A utensil which has holes, cracks, or crevices in it, must be cleaned thoroughly prior to hagala (שו”ע או”ח תנא:ג-ה). One should pay particular attention to the connection of the handle to the body of a pot, and to the benchmark stamps on silverware. If there are particles of food left behind, the hagala is invalid. Note, this only applies to hagala, not to libun. All rust must be removed prior to hagala. However, stains in the metal do not have to be removed. Red splotches left behind by rust, or burn marks, are not problematic.
Knives, especially serrated knives, should be cleaned very well with steel wool before hagala. This is learnt from the halachot of using a non-kosher knife for cold. The Shulchan Aruch and Rama (יו”ד קכא:ז) write that knives that are purchased from a non-Jew may be used for cold as long as one first does ne’itza – stabbing the knife in hard ground – ten times. This removes any oils and residues that may be left behind on the knife. Knives are often very difficult to clean properly, therefore they require a higher level of cleaning.
Regarding hagala, there is no difference between the forbidden residue that one must remove for cold usage and that same residue when it comes to cleaning a knife for hagala. That is why one must be extra careful when cleaning a knife before doing hagala. Just as when using it for cold, the knife is not considered clean until one does ne’itza, so too for hagala the knife is not considered clean without ne’itza. The general consensus among the poskim is that using steel wool is equal or better than ne’itza.
How to do hagala
When doing hagala, the water should be bubbling in a pot that is still on the fire. When doing hagala on many utensils one after the next, one should be careful to wait long enough between immersing each utensil until the water in the pot starts bubbling again. The utensil does not have to be immersed in the boiling water all at once. One may kosher a utensil one part at a time. When koshering multiple utensils at once, one should take care to ensure that each one has space around it, and that they are not touching each other.
If one is koshering for Pesach, and is koshering in a pot that was used for chametz, they should first kosher the pot itself before using it to kosher other utensils. One can kosher a pot by filling it with water, and when it starts bubbling, drop in a hot rock to make the water flow over the top edge of the pot.
One should preferably leave the utensil that is getting koshered in the bubbling water for a few seconds (ט”ז או”ח תנב ס”ק ב). If the utensil was removed from the boiling water immediately, the hagala still works. After the hagala one should rinse the utensil in cold water (תשובות רש”י רנט, שו”ע או”ח תנא:ז).