I have done remodeling for 37 years & finished drywall and repaired plaster using drywall methods for 35 years. There is an unsolved problem that comes up unpredictably on painted drywall or plaster that remains unsolved and unexplained. I call them "hot spots". When the mud (I use only All Purpose) is applied there is a chemical "bubbling" resulting in scattered "pores" in a shotgun pattern. This can be minimized by priming, but not always eliminated. I called USG once and the technician diagnosed it as either outside air infiltration or air in the mud. From the locations and the experience itself I can't accept that answer. There appears to be a chemical reaction going on and I have suspected (though not tested) something like aluminum oxide from previous sanding (we sponge). We don't have this problem on new drywall.
Does anyone have the answer to this? I would appreciate it.