Then either boiling hot water should be poured on them, or they should be completely covered so that nothing Pesach dik touches them. Some people do both. Wash them with a household cleaner. The usual practice is to cover the tables. Wash the top and side surface areas with a rag soaked in a household cleaner.
Some people cover it with aluminum foil.
Old grates can be kashered by first cleaning them and then lighting all the burners, raising them to their maximum heat, and preferably putting on a blech while the burners are on. This spreads the heat over the whole top and intensifies the heat on the grates. Let it burn for 5-10 minutes. Then heat the inside of the oven by turning the oven on the highest temperature for about one hour. However, if a closed oven insert for baking and roasting is available, this would be preferable.