Originally Posted by mayhem69
so are you guys saying i should apply a primer over the existing semi-gloss first??
That's kind of a toss up.
Officially, according to the Paint Quality Institute,
If you're painting a latex paint over anything glossier than a satin latex paint, you should sand the old paint down to ensure good adhesion of the subsequent coat of paint.
Primers, however, are made with binders that are chosed because they stick better to smooth surfaces. So, a primer will stick better to a semi-gloss paint than a paint will.
However, latex paints form films by a process called "coalescence", and that coalescence requires a special solvent that causes the plastic binder resins in the new paint to become soft and sticky. Typically, that special solvent (called a "coalescing agent") ALSO causes the surface of the old latex paint to become soft and sticky too, so the two paints stick well to each other. However, there's no guarantee that the coalescing agent in the new paint will soften the surface of the old paint and make it sticky, so you can't count on new latex paint bonding well to old latex paint. You can with flatter paints because even if the old paint doesn't get soft and sticky, it's roughness results in it having a larger surface area, and the same adhesion over a larger area means a stronger bond.
You're right in the transition zone. "Satin" is the supposed "cut off" point where you should sand prior to painting. The problem is that every different paint company has a different idea of how glossy "satin" is. Behr Paints, for example, have an "eggshell" that's downright "FLAT" and some companies have semi-glosses that have about the same sheen as other company's semi-gloss. So, there's no concensus as to how glossy "satin" is. It's kinda like a recipe calling for a "pinch" of garlic, but not saying how big a "pinch" is. The bigger your fingers, the bigger a "pinch".
If it wuz me, I would paint some of your Zinsser's Permawhite on your semi-gloss paint, allow it to dry completely over 2 or 3 days, cut a checkerboard pattern a little less than 2 inches by 2 inches through the Permawhite paint with shallow pressure on a razor, apply some 2 inch wide masking tape to cover the cut pattern in the Permawhite, press the tape down, and then quickly pull the tape off. If more than 3/4 of the paint stays stuck down, or pulls the existing paint off with it, that means you're getting good adhesion to the underlying paint, and you can just paint over paint. If less than 3/4 of the Permawhite stays stuck down, you're not getting as good adhesion as you should, and you should either sand the existing semi-gloss or apply a primer before painting. If you choose to apply a primer, and if it wuz me, I'd use an interior alkyd (oil based) primer only because it'd be less permeable to humidity in the air so that the humidity in the bathroom would stay in the bathroom to be removed by the fan rather than permeate throughout the house.
PS: The checkerboard pattern is how commercial labs test the adhesion of paint (and other coatings).
Alternatively, just paint a small area of your semi-gloss with Permawhite, allow to dry completely and then try scraping it off with a sharp paint scraper or even a fingernail. If it puts up a good fight to stay on, and makes you work to get it off, then you're getting good enough adhesion.