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A vessel or surface that is not Kosher, or was used for Chametz, must only be Kashered if food will come in direct contact with it, or will come in contact in a way that it can absorb taste from it, such as through liquid. Thus, if one does not desire to Kasher a certain item, or the item is not Kasherable, then it suffices to properly cover the item with a material that will obstruct any transfer of taste. This can be accomplished by placing a sheet of metal [such as tinfoil], or other material [such as PVC, or a plank of wood] over the surface. [The material may be of any thickness, as long as it will not move or tear. The surface must be cleaned of any visible food or Chametz prior to placing down the metal, and it must be dry. Likewise, throughout its use, one must make sure that water or other liquids do not enter in-between the covering and the surface if he desires to ever place hot Pesach food directly on it. Practically, the suggested advice and custom is to Kasher all surfaces with Iruiy Keli Rishon and Even Meluban, including even surfaces made of material that cannot be Kashered. One is then to dry the surface very well and cover it with PVC or a thick piece of tinfoil that will not tear. One is to glue, tape, hammer, or staple the material onto the surface so it does not move. In addition to all the above, throughout Pesach it is proper to be careful not to place any hot food, or hot pot, directly on the covering, and is rather to place it on a trivet and the like.]
Bed sheet: A [thin] bed sheet is not a valid interval to allow placing hot foods on a surface that has not been Kashered or that contains Chametz. Furthermore, even cold foods may not be placed on it during Pesach if there is Chametz under the sheet, on the vessel.
All surfaces, including surfaces that cannot be Kashered, may be used over Pesach if they are covered properly. The order for achieving this is as follows:
1. Pour hot water from a Keli Rishon and Even Meluban onto the surface.
2. Dry the surface completely.
3. Cover it with a thick piece of tinfoil, PVC, or wood.
4. Fasten the covering material to the surface using tape, glue, staples and the like.
5. Avoid placing hot foods or hot pots directly on the surface throughout Pesach, and is rather to place it on a trivet and the like.
Should one cover surfaces with tinfoil, PVC, and the like even if they have been Kashered?
From the letter of the law, once these areas have been properly cleaned and Kashered they may be used for all foods without any cover. However, some Poskim rule that one is to cover the surfaces even after they are Kashered due to suspicion that perhaps they still contain actual Chametz that was not properly removed. Practically, the widespread custom is to cover all items that contact food even after they have been cleaned and Kashered.
 See Admur 451:65; 67; Rama Y.D. 92:8; Taz Y.D. 92:29; M”B 451:34; Nitei Gavriel 77:3 footnote 7
 See Maharil Hilchos Hagala p. 21 “Or with a thick blanket or piece of wood”
 Admur 451:65 “A piece of iron, or other material which one is not using on Pesach”; See also Taz ibid
 Implication of Admur ibid and Taz ibid who do not mention any measurement; See however Admur 451:48 and 51 and 65 who invalidates a sheet and Admur 461:2 who rules that a new clay floor must be 2 centimeters thick, nonetheless one can differentiate between the cases, as a sheet is not a true cover, while the clay becomes attached to the surface and hence becomes part of it.
 If there is Chametz food, or any moisture between the surface and cover, the entire allowance that the cover provides is invalidated. See Admur 451:67; Chochmas Adam 45:19; Sheilas Yaavetz 1:113; Chavos Daas 92; Hakashrus 1:20; Pischei Halacha 1:30; Igros Moshe Yoreh Deah 1:40 and 59 regarding grates
 See Rama Y.D. ibid and M”B ibid that Lechatchila one should be stringent regarding relying on the “two pots touching each other” allowance, and hence, if it is Kasherable, one is to Kasher it. This especially applies regarding Pesach. However, see Admur 451:65 who permits placing a metal sheet a Chametz surface even initially without Kashering it. Some prove from here that it is thus allowed even initially. [Arugas Habosem 119; Mishnas Sachir 57]
 As many times it is Kasherable according to some opinions, and doing so certainly makes no harm.
 See Poskim brought in Q&A regarding why we cover it even if it was Kashered.
 This is due to suspicion that perhaps water or other liquids have traveled into the area underneath the cover, between it and the Treif surface, and if a moist hot food/pot is then placed on top of the cover, it can forbid everything there. [See Poskim ibid] Nonetheless, Bedieved, everything remains Kosher as we follow majority usage. [Admur 451:27 and 72]
 Admur 451:48 and 51 and 65
 See Maharil ibid
 Michaber 451:20; Admur 451:58
 M”A 451:38 in name of Maharil [even regarding tables]; Elya Raba 451:40 [only regarding shelves, not tables]; M”B 451:115 [brings both opinions]; Dvar Moshe Tinyana 98:36; See Nitei Gavriel 77:1 footnote 2
Does this law apply even to tables and counters or only to shelves? The M”A ibid applies the ruling of the Maharil to all surfaces that contact food, including tables. However the Elya Raba ibid limits this stringency of the Maharil to cabinets with shelves in which it is difficult to clean the corners of Chametz, and hence it is to be covered. However, a flat surface such as a table, even the Maharil agrees that there is no need at all to cover it. The M”B ibid brings both opinions and seems to side with the ruling of the Elya Raba. So also rules the Aruch Hashulchan 451:41. Nevertheless, it is customary of Jews, including Anash, to be stringent in this matter.
Opinion of Admur: Admur ibid completely omitted the ruling of the Maharil; M”A and Elya Raba. He only mentions the necessity to cover an item in 451:48 regarding vessels made of wood that are constantly used with dough and flour; and in 451:65 regarding an oven that was not Kashered. No mention is made anywhere regarding covering an already Kashered table or cabinet. Nevertheless, it is customary of Jews, including Anash, to be stringent in this matter. [See Nitei Gavriel ibid]
 M”A ibid; Piskeiy Teshuvos 451:44