What is the minimum amount of dust that can be left on a wall before it adversely affects primer application? If it has to be completely free of dust, how do you get it all? Taking a damp rag over the wall seems to be the way to do this based on my own research, but then you have to rinse it wring it out. This dirties up the water which just means more particulates would end up back on the wall as dust after the water evaporates. Plus, going over all walls and ceiling of a 250 sq ft room with a damp rag just seems like a PIA. Is there a better way?
I picked up a mop with a wringable, rectangular sponge head from the drug store. What I did was get it wet and wring it out to dampness. After taking it from the ceiling to the floor and stepping to the side a bit to repeat, I'd be able to get about three sweeps of the wall before the head looked like it needed to be wrung out again. I did all walls and ceiling of my 250 SF room in well under an hour.
There is now only ever so slight of a trace of dust on the surfaces. It may be ready to prime right now, but, since it was so easy, I'll probably do it once more tomorrow for good measure. It will go even quicker this time since there will be far less wringing out of the sponge head.
I've heard from professional drywallers/painters that they don't even try and take all the dust from sanding off, just a quick sweep of the walls to get the majority off. The really fine drywall dust will add itself to the primer coat and help smooth it out. I tried this approach when I remodeled my living room and the results were great.
I recently prepped two rooms for primer and paint. I read that the best prep is to use water and sponge as you did. It was not so much the drywall compound dust, as the thin coating of kaolin the drywall manufacturer uses to keep sheets from sticking together that the water will take off.
I currently have a garage ceiling to scrape, mud, and prime, that pro painters apparently didn't prime. The paint is peeling off every where there is a joint, and will peel off the surface with a quick swipe of a 5-in-1 tool. Bozos!