i have vinyl coated drywall interior walls. i want to make them paint ready and did other walls within the home by skim coating/whiting out the walls with drywall mud. i did have a problem with bubbling of some of the paint. I was recently told by a painter to use shellac/primer. the question is, do i use this before applying the drywall mud? i am told the shellac is a moisture barrier and the rooms im doing are master bedroom and master bath, which will have moisture. does anyone have any ideas of the best way to convert vinyl coated drywall to regular drywall, paintable walls?