How to fix window sill damaged from water & humidity?

peterj01

Hi,

This is Peter. I need some advice on how i can fix my bathroom window sill which is just above the bath. The paint has peeled and the wood looks a bit swollen . Please help before it gets worse or if I need to get the whole thing removed and replaced. I have attached the pictures Thankyou

MitchellMc

Welcome to the Bunnings Workshop community @peterj01. It's wonderful to have you join us, and many thanks for your question about window sills.

Thanks for posting the photos and the details, which definitely help, and you are right to tackle this sooner rather than later.

From the photos, the windowsill and the lower timber surround appear to be MDF. Unfortunately, MDF is not a great material for bathrooms, especially on a window above a bath where humidity is high, and the surface is regularly exposed to moisture. Once MDF starts absorbing water, it swells and breaks down internally, which is exactly what you are seeing with the peeling paint and puffed-up edges. Because of that, the sill itself will almost certainly need to be replaced rather than repaired.

The good news is that it looks like the vertical sides of the window frame have not been affected as badly. If they are still solid, they could likely stay. Those areas can be lightly sanded back where the paint is lifting, then reprimed and repainted. The key with any timber in a bathroom is having a continuous, well-sealed paint layer so moisture can never get to the substrate.

With the sill, there is not really a reliable long-term way to save what is there. Once MDF has swollen like that, the damage usually runs through its thickness. You could sand it back to remove the raised areas, and skim coat it with something like builder’s bog, then sand, repaint and reseal it to the window frame. That might improve how it looks in the short term, but it is unlikely to last, especially in that location.

The better option would be to replace the sill with solid timber. Solid timber copes far better with occasional moisture and, when properly sealed, will last much longer in a bathroom environment. When it is installed, make sure all faces are primed and painted, and that the joint between the sill, window and the window frame is well sealed so any water running down the glass lands on painted, protected surfaces rather than soaking into the timber.

I would recommend engaging a handyperson or carpenter to replace the sill, as it is a fairly common repair and not usually a major job. It would also be worth checking your bathroom ventilation at the same time. Improving airflow with a good exhaust fan will help reduce moisture buildup and go a long way towards preventing the same issue from happening again.

Please let me know if you have any questions.

Mitchell