Painting Bathroom? How to prepare the walls for new paint?

Wash the walls with TSP. Let dry thoroughly. Use a good primer like Kilz or Zinsser, then paint with a semi gloss or gloss washable paint for easy cleaning.

The quickest and easiest way would be to paint them with Kilz 2 first. It adheres to virtually any paint or wall material and will cover stains or glossy paint easily. Then your wall will be ready to accept your new paint with no issues. With Kilz 2 you can actually just have the Kilz 2 tinted to the color of your choice and then paint it straight on the old semi gloss you have now. Some people don't trust this way so they go with the pre painting so they're more comfortable that it'll get full coverage. Just wanted to give you both options. Hope this helps.

Remember,After complete bathroom painting, you can't taking shower. You should follow this article.

https://bathadvice.com/how-long-to-wait-to-shower-after-painting-bathroom/