I decided to paint the downstairs bathroom. A girlfriend insisted on helping and was predetermined on saving me money(so she says, now I think she was trying to sabotage) by bringing me paint that she had left over from a remodel. Well when my brother-in-law opened the paint(by the way it was a I think 5 gal container)he was like "WOW, that smells really bad, like water dripped or had gotten in the container." She swore up and down that it wasn't bad, yada, yada, yada..Now my bathroom smells soooo bad, like mold. Only two walls have been painted. But that's two walls too many. So my question is how and the hell do I get rid the smell, because it's been about three weeks and it still smells. Do I need to strip the paint? I even tried covering it with a new color and you can still smell it. HELP!!!!
Short answer: It will have to air out for weeks to months.
It may be possible to heat the room up as hot as you can get it to speed the out-gassing process.
Moral of the story: NEVER paint with old sour paint.
If near a term solution is needed, you could power abrade (electric sander) the paint off. Of course that means it will really stink in process.
Then you will have to re-texture the bare surface, paint primer, and paint. It is a fair amount of work, especially if there are obstructions: the worst being a toilet you don't want to remove.
But in about two days you can have all the stink gone. Otherwise, alas, you'll have to wait months for the stink to dwindle naturally.
Please - take the paint off. Fresh interior latex is much easier to remove than you think. I work with 2-part urethanes... those are hard. I'm not saying it won't be work - it will - but it's your least bad of several options.
Others have had this same problem. example example example example example Waiting did not work for them. Heat did not work. They tried painting over with other paint, but struggled to find a sealer that would be effective.
You also don't want to overpaint known-bad paint, because now you have a bad layer. When that layer fails (not at once) the you'll have peeling and blistering for years - and that will bring the smell back. I have a project where full stripping is not feasible and we just have to paint it every year to fix the blisters.
Seriously, removing the paint now before it cures fully is the path of least pain.
I remove latex by keeping it wet (water is its solvent) while scrubbing. I would use the least stiff brush that does the job, so as not to mar the layers below too badly. But I would rather gouge the lower layers than leave any horror paint on the wall. It will make the next layer fall off. You can always resurface the walls flat with sanding primer and power sanding if needed, which it may not. With luck the new paint will come off clean.