Let’s assume you are focusing on problem areas and not painting an entire bathroom. The brush vs. roller debate is more of a brush + roller conversation. The quick answer to your question is: both.
Roll across space & brush in spots
You could use your brush for every surface, but the roller will be a much quicker and more consistent application across large flat surfaces. Use your roller where you can, and use your brush where you need: corners, trim, around the fixtures, and behind the toilet.
Before paint, comes prep
Before the paint goes on, be sure to thoroughly remove those peeling bits from the wall. This goes for quick touch-ups or redoing entire bathrooms. Fixing the underlying problem is the only way to ensure against the same problem occurring again.
Try painting with these steps:
- Use a paint scraper to clear the surface of any imperfections.
- Patch any cracks with putty and scrape off excess with scraper.
- Sand down putty to create a smooth, flat surface between your now-bare spot and surrounding paint.
- Clean sanded areas with a rag.
- Paint your surface with a Paint and Primer in one, like Olympic ONE® Interior Paint.
Let dry, check for seconds
You may need a second coat, so check your targeted areas or walls after the recommended drying time has elapsed. Whether you use a brush, a roller, or both, the end result should be a freshly coated portion of your room that has become invisible once dry.