One must make certain that there is nothing separating between the vessel one is immersing and the waters of the Mikveh. Thus, when one is immersing a vessel, one must hold the vessel loosely, for if one holds it tight, one’s hand will be separating between the vessel and the waters of the Mikveh. Nevertheless, if one wets one’s hand with some water before immersing the vessel in the Mikveh, there is no concern of a separation even if one holds the vessel tight, for the waters of the Mikveh are touching the water on one’s hand and will be in essence connected to all parts of the vessel.
When immersing new vessels, one must make sure to remove all stickers and the like which can create a separation. (Nevertheless, not all things found on the vessel create a separation and only things which one makes sure not to leave on the vessel create a separation, similar to the laws of a separation during Netilat Yadayim and the immersion of a Niddah in the Mikveh. Therefore, one should always make sure to remove all stickers before immersing the vessel. Even stickers which say “Crystal” which add prominence to the vessel should preferable be removed before immersing the vessel according to Maran zt”l.)
If there is rust on the vessel, one must clean or scrape it off well before immersing it. Similarly, a pocketknife with the blade folded into it must be opened before immersing it in order for the water to be able to reach all around the knife.
If one buys a new set of silverware, immerses it in the Mikveh, and later finds out that one piece of flatware has not been immersed and is now mixed up with the rest of the set, some say that one is not obligated to immerse all of the vessles again while others say that all of the utensils must be immersed once again. According to Maran Rabbeinu Ovadia Yosef zt”l, they must be immersed again without a blessing since immersing vessels is a Torah commandment and when in doubt, one must act stringently; only regarding the blessing do we say that one should act leniently and not bless, for acting stringently by reciting the blessing would cause one to transgress the prohibition of uttering Hashem’s name in vain, as the rule goes, “When in doubt, do not bless.” Nevertheless, if the vessels in question are glass, one may act leniently and one need not immerse them again since immersing glass vessels is only a rabbinic law, not a Torah commandment. Regarding rabbinic laws, we follow the rule, “When in doubt regarding a rabbinic law, one may act leniently."
Although even outside of Israel there are many Jewish-owned factories which manufacture vessels and dishes, the rule is that one must assess the majority of factories in order to determine whether or not the vessels require immersion. Since it is known that most factories manufacturing vessels and dishes outside of Israel are under non-Jewish ownership, any vessel produced outside of Israel must be immersed in a Mikveh and the blessing is recited as well. Although the rule is generally “When in doubt, do not bless” and since we are still in doubt whether the owner of the factory is Jewish which would make the vessel not require immersion and the blessing recited on such an immersion a blessing in vain, nevertheless, Maran zt”l writes that since we have a clear Torah law obligating us to take the majority of factories into consideration, thus, immersing these vessels is an absolute Torah law for which one must recite a blessing.