How to paint over dirty ceilings/walls.

When I painted our bathroom I didn’t wash the ceiling and did just a quick wipe down before painting with Behr paint/primer combo. However, if they paint in your bathroom was oil based, you’ll have to prime with an oil based primer first if you’re going to use latex paint, otherwise your paint will peel and crackle

Wash thoroughly with TSP. Use a paint formulated for kitchen and bathroom use and at least one coat of a premium quality primer. Yes it is a bit of work, but the end result will be more professional and last far longer than just slapping a coat of random paint on dirty walls. My personal preference is a stand alone paint store rather than big box home improvement store. Painting is a chore - make the end result count so you can be proud.

My guy used produce call Blanket, covered a walls and ceilings in my home before painting. Smoker had lived in house!

I agree with Naomie. Wash everything down. Dirt and grime is a barrier for the paint. Definitely prime. Your paint color will be more true and vibrant. For the bathroom use a bathroom paint. Has a mildewcide in it.

I have taken a sponge mop with warm water and vinegar, and it was really fast and easier on my back. Because I didn't use soap, I didn't need to rinse it. It does need to dry thoroughly though to avoid any chemical interaction. Then I prime, unless I am using the same colour.

Use a mildew-resistant paint for bathrooms. I have never had any problem since I have done that. Even my bedroom ceiling used to get mildew spots in the corners from humidity, and it hasn't been an issue in over 20 years after using a mildew-resistant primer.

https://www.consumerreports.org/interior-paints/mildew-resistant-paint-for-your-bathroom/