Repairing Nail Pops

I've repaired "nail pops" in my bathroom numerous times. I wait until the repair is dry, sand, and paint (latex). It usually looks great.... until a month later when the repair cracks. The bathroom is used for showering, so it gets mighty humid there. Do I need to use special filler? I've tried various "spackling compounds," even outdoor ones in hopes that they would be more moisture proof. Or should I seal the repair somehow?

You appear to have done the cosmetic repair correctly. What you haven't done is solve the underlying problem... the wall is moving! First, a nail pop primer... nail pops are visible dimples in the wall over the wallboard nails or screws. This is caused by movement in the wall due to a space between the wallboard and the wood wall studs. This can happen for two reasons... shrinkage in the wood or improper nailing/screwing. This is a common and annoying problem during the first few years in new construction, so much so that it has become common for contractors to use construction adhesive on the studs to keep stud and wallboard blissfully together!
Though some nail pops proudly display themselves without warning, many lie in wait until someone leans on the wall... oops... or during routine repairs such as picture hanging or even painting. The wall suddenly moves, causing the nails to push on the wallboard compound covering them. The pops appear like mushrooms on a damp lawn... as if you didn't have enough work to do!
Simply repairing the visible wall damage... the "pop" or dimple caused by the movement of the wall... is not enough. The pop will reappear... guaranteed... unless you take steps to tighten up the wallboard. Some folks think they can take the easy path and simply bang in the nail or tighten the drywall screw. Sorry... usually not enough. The best repair is to install a drywall screw three inches above and below the pop while pressing the wallboard against the stud. This action both tightens the wall and gives support to the weakened drywall around the pop.
Be prepared (not scared, just prepared) for a few more nail pops to appear along the same stud (or even in adjacent studs) as you do this repair. By disturbing the wall, you are arousing the nail-pop demon! Boogah! Boogah! Get this little devil under control by doing the same aforementioned procedure on all the subsequent pops and I can give you a 99% guarantee that this is one home repair you will not have to repeat!

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