Sticker residue and tevilat keilim

I brought a drinking glass to the mikvah and removed the sticker, but afterwards notice there was a little bit of sticker “gunk” or residue left on the glass. It really was not much at all and is hardly noticable, but is this an issue? Does the glass have to go back to the mikvah?

Only if you won’t use it without cleaning

What about an older kli that one had not dipped initially before using? If there’s calcium buildup and other such stains that are cooked on or difficult to remove, to those pose a problem for tevila?

Again, if you would use it as is, its not a problem.