Hi @Xi,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Moisture will certainly play a part, being in a bathroom, but seeing the severity of the issue will help to direct our approach. It may be a simple case of managing the mould and then painting over it, or it could mean stripping back the paint, dealing with the mould, repriming and then painting with a ceiling paint with mould-preventing additive. It all depends on the circumstances, so seeing it will be a great help in determining the approach.
Have you got any photos that you could share? If you need a hand with uploading photos, please let me know.
It does sound like the ensuite has been struggling with excess moisture for a while. The rusting sprinkler head and the mould forming around the ceiling edges are both signs that humidity is not being removed fast enough. Even when the fans are running longer, they might not be moving enough air, so it can help to check the actual suction by holding a piece of toilet paper over the grille to see if it sticks. If the fan has weak suction, the first step is to remove the cover and clean dust from the blades and the ducting, since buildup can drastically cut airflow. If suction is still poor, the duct may be blocked or the fan may be undersized for the room, in which case clearing the duct or upgrading to a higher capacity exhaust fan is usually needed to keep moisture under control. If there is a window, opening it during and after showers will also improve airflow and drastically reduce moisture in the room. Additionally, keeping the fan on for at least twenty minutes after each shower also makes a noticeable difference.
For the mould that keeps returning, it is fine to sand those areas, but only after treating them. Clean the mould first with a mould cleaner, following the instructions closely, then rinse and allow the surface to dry. Once the mould is neutralised, you can give it a light sand to remove any roughness, just ensure you wear a mask to protect yourself. A light sand will help remove the remaining residue and give you a good base for repainting. Ensure you wipe away any dust after sanding.
With flaking paint, it is likely that you will have to prime the ceiling in some capacity, so using a mould inhibiting primer like Taubmans 1L Advanced Adhesion And Mould Blocker or Dulux 1L PRECISION Stain & Mould Blocker, then painting over it with a bathroom ceiling paint like this Dulux White Ceiling +PLUS Kitchen And Bathroom Paint or a standard ceiling paint with Protite Mould Defender Paint Additive would be the way to go.
Hopefully, this answers a few questions. I will happily assist further if you have further information or photos that you can provide.
Jacob
