Gemara

(Mishnah): During the Regel we warn the Amei ha'Aretz Kohanim not to touch the Shulchan (for we cannot immerse it later).

R. Eliezer says, all Kelim in the Mikdash must be immersed (after the Regel) except for the gold Mizbe'ach and the copper Mizbe'ach, for they are like land;

Chachamim say, because they are plated (this will be explained).

(Gemara) Question: We should say (that the Shulchan is Mekabel Tum'ah) because it is plated!

(Mishnah): If a table caved in or was covered with stone (which is not Mekabel Tum'ah), it is Tamei only if there remains an intact place (not covered by stone) to hold cups;

R. Yehudah says, it is Tamei only if there remains a place to hold pieces (of bread or meat).

Answer #1: Perhaps the Shulchan is not Batel to its covering, for it is made of cedar wood, which is important!

Partial rejection: This answers only for Reish Lakish, who says that only Kelim of cheap wood are Batel to their covering, but Kelim of expensive wood are not Batel. It does not answer for R. Yochanan, who says that even Kelim of expensive wood are Batel!

Answer #2: Perhaps this law (a Keli is like its plating) is only if the plating is nailed on (unlike the gold on the Shulchan).

Rejection: Reish Lakish asked whether or not this law is only if the plating is nailed on, and if it is only if the rim is covered. R. Yochanan answered that it does not depend on either of these.

Answer #3: The Shulchan is an exception, for the Torah calls it a Keli Etz - "Ha'Mizbe'ach Etz".

27a (Mishnah - Chachamim): Because they are plated.

Question: This is a reason why the Mizbechos should be Mekabel Tum'ah!

Answer #1: Indeed, Chachamim mean that the Mizbechos are Mekabel Tum'ah because they are plated.

Answer #2: Chachamim argue with R. Eliezer. They say, you are Metaher because they are like land, but if not you would be Metamei because they are plated. This is wrong. The plating is Batel to them!

Kelim (11:4) A bolt is Tamei, and Metzupeh (plated) is Tahor.

Rishonim

Rashi (Chagigah 27a DH Mai): Also in Answer #2, Chachamim argue with R. Eliezer. He holds that the Mizbechos should be Mekabel Tum'ah because they are plated. They are Tehorim because they are like land. Chachamim say that the plating is Batel to them, so in any case they are Tehorim.

Rambam (Hilchos Mishkav u'Moshav 11:11): On Motza'ei Yom Tov after the Regel all the Kelim in the Mikdash require Tevilah and ha'Arev Shemesh, except for the gold Mizbe'ach and copper Mizbe'ach, because their plating is Batel to them.

Kesef Mishneh: This is like Chachamim, according to the second answer, for the Mishnah connotes like it. Also, the latter answer is (normally) primary.

Mishneh l'Melech: Even though plated Kelim are not Mekabel Tum'ah, the Torah calls these "Etz".

Rambam (Hilchos Kelim 4:4): The Chipuy (cover) of a bed and all Chipuyim are Tehorim, whether they are of wood, bone, leather or metal. "Asher Ye'aseh Melachah Bahem" excludes Chipuyim. Similarly, if a Keli of wood or bone can hold something and it was plated with metal it is Tahor, since the plating is Mevatel them. The plating itself is Tahor.

Rebuttal (Ra'avad): What is the source for this?! Just the contrary, if a Keli cannot hold anything and it was plated with metal, this causes that it is Mekabel Tum'ah! We sometimes find that the Keli is Batel to the plating.

Perush ha'Mishnayos (Kelim 11:4): A wooden bolt plated with metal is Tahor. We learned from the Mizbechos that a Keli Etz plated with metal is Tahor, even if it can hold something. The plating is Mevatel the Keli.

Objection (Rosh (Mishnayos Kelim 11:4) and Kesef Mishneh): The Rambam learns from Chagigah that plating is Metaher. Just the contrary! Rashi explains that plating is reason to Metamei! R. Shimshon explains the Mishnah of the bolt differently. If the bolt was made of Tahor metal and it is plated with Tamei metal, it is Tahor, for the Ikar Keli determines the law.

Tosfos Yom Tov (Kelim 22:1 DH O she'Chifan): R. Shimshon holds that the Keli is Batel to the plating if they are of different materials, and oppositely if they are the same (but one is Tamei and the other is Tahor). The Rambam holds that plating is never Mekabel Tum'ah; it is always Metaher the Keli.

Answer (Ri Korkus, brought in Kesef Mishneh): The Rambam explains the Gemara differently. On 26b, it asks why we said that the Mizbechos are Tehorim because they are stationary. In any case they are Tehorim because they are plated! We know from Sifra that plating is Tahor. The Gemara answered, since the verse calls them Etz, we do not follow the plating. However, the discussion of the Seifa is difficult for the Rambam. He is forced to say that even though the first answer holds that plating is a reason to Metamei, the latter answer holds oppositely.

Question: If so, why does Answer #2 say that Chachamim address R. Eliezer according his reasoning? It should explain simply that Chachamim Metaher because the Mizbechos are plated!

Answer (Mishneh l'Melech Kelim 4:4): Since the Mizbechos themselves (without the plating) are not Mekabel Tum'ah, it is not proper to say that they are Tehorim due to the plating.

Tosfos (Chagigah 27a DH Ibo'is): R. Eliezer holds that if the Mizbechos were not like land, we would learn from the Shulchan they are Batel to the plating and are Temei'im. Chachamim hold that a priori we would say that a plated Keli is Mekabel Tum'ah. Since a verse is needed to Metamei the Shulchan, this teaches that where there is no verse, e.g. the Mizbechos the plating is Batel.

Rambam (6:15): If a table was covered with stone, it is Tamei only if there remains an uncovered place to put cups. If it is totally covered it is Tahor, whether or not the plating is firmly secured, whether or not the rim is covered, and whether or not the wood is precious.

Mishneh l'Melech: The Ri Korkus says that even if it was covered with metal, the plating is Metaher the Keli.

Rambam (10:11): A metal bolt is Tamei. If it is wooden and plated with metal, it is Tahor.

Mishneh l'Melech: R. Shimshon is Metamei a wooden Keli plated with metal (see Tosfos Yom Tov above).

Poskim

Shulchan Aruch (YD 120:1): If one buys from a Nochri a Keli for food that is plated with lead on the inside, he must immerse it.

Rema: Some say that if it is plated with lead even on the inside he immerses it without a Berachah. This is the custom.

Gra (16): Even though regarding Tum'ah Chachamim say that the plating is Batel to the Keli, here it is not because the plating (touches the food when the Keli) is used.

Maharam (Teshuvah 3:20 DH v'Hinei Mah): The Rambam holds that everything depends on the plating. However, most Poskim disagree. Therefore, one must immerse an iron Keli plated with something that does not require Tevilah.

Noda bi'Yehudah (OC 1:1 DH v'Hinei ha'Beis Yosef): In Chagigah we learn from the Shulchan and the Mizbe'ach that regarding Tum'ah the Kelim is Batel to the plating. This is because all the usage in on the plating. However, plating on Tefilin is Batel to the Tefilin, so it need not be from something permitted to eat.

Shulchan Aruch (OC 629:2). One may not use something that is Mekabel Tum'ah for Sechach.

Sha'arei Teshuvah (3): The Eliyahu Rabah says that the Maharil once saw a piece of wooden Sechach with a metal band around it and commanded to take it down.