Chatzitzas on vessels:[1]

When immersing a vessel in a Mikveh one must remove all invalidating intervals from the inside and outside of the vessel in order for the immersion to be valid. If there was an invalidating interval on the vessel at the time of immersion, the immersion is Rabbinically[2] invalid and the vessel must be reimmersed with a blessing [if an interval was made from the previous blessing[3]]. This applies whether to the inside or outside of the vessel, and applies whether the vessel is made of metal or of glass.[4]

Definition of an interval:[5] Any item which common for people to be particular against having on the vessel is considered an interval [even if it only covers a small amount of the vessel]. An item which is common for people to retain on the vessel [and the owner is likewise not particular in this matter[6]] is not considered an interval unless it covers majority of the vessel. [If it is common for people to be particular against having it on the vessel, but the owner is not particular, it is nevertheless considered an interval. Thus, only if both the owner and most people are not particular is it not considered a  Chatzitza.[7]]

Soot:[8] The soot that becomes attached to the outside of a pot is considered like the wall of the pot, being it is common to be there, and is thus not considered an interval. [However some Poskim[9] are stringent in this matter. If however it is found inside the vessel, then it is considered an interval if people are particular in this matter.[10]]

Rust:[11] One must remove all rust from the vessel prior to immersion. If one did not remove the rust, then if he is particular against it being on the vessel, it is considered an interval and the immersion is invalid. If one is not particular against having it on the vessel [and it only covers minority of the vessel], it is not considered an interval and the immersion is valid. If one removed the rust to the best of his ability, and some rust still remains which cannot be removed through the normal methods, then it is not considered an interval.[12] Thus, the rust color that remains on vessel and has no substance is certainly not considered an interval.[13]]

Handles of vessel:[14] The handle of a vessel is not considered an interval and does not need to be removed prior to immersion. If however one plans to eventually remove the handle, such as if the handle was not placed in properly, or is broken, and hence must be removed and replaced or reinserted[15], then it is considered an interval and must be removed prior to immersion. [If however one plans to permanently leave the broken or improperly placed handle within its socket of the vessel, then it is not required to be removed prior to immersion.[16]]