I have almost finished my first drywalling project, and things have gone fairly well, under the circumstances. However, I have one problem -- after putting the last (3rd) layer of mud on an inside corner, a 2-3"-long bubble appeared under the drywall tape. I suspect the cause was overzealous sanding of layer 2 mud - true? Anyone have a suggestion for a fix?

Thank you!!

 

This is called a blister. It is caused by not having enough mud under the tape. Simply cut out the blister and cover with mud. If the blister is really big put another piece of tape over the cut out blister and finish