Hi @Fiona_S,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
I'll start by saying that any time you are working around glass, you should put safety at the forefront of your mind. Wear gloves, long protective clothing and enclosed shoes as there is always some potential for the glass to shatter.
I'd say you're right that moisture has pooled on the horizontal parts of the frame, likely from condensation forming on the glass, dripping down and pooling on the horizontals.
To fix the rust, the first step is preparation. Before you begin, protect the glass and surrounding areas with painter's tape and plastic drop sheets, and put on gloves, safety glasses and a mask. Rust removal creates a lot of fine dust, and you’ll want to keep that contained, so adding a drop cloth beneath the windows can also help with cleanup.
Once everything is masked, start removing the loose rust and paint. A stiff wire brush or sanding block should do the trick. The aim is to get back to clean metal where possible. If you can’t reach every crevice or remove every last trace, a rust converter is a handy option. It reacts with the remaining rust and stabilises it, giving you a good base to work from.
After the surface is back to sound material, wipe everything down with methylated spirits to remove dust and oils. Once clean and dry, apply a coat of anti-rust metal primer such as White Knight Rust Guard Grey SLS Etch Primer Spray Paint. This stage is important, as it locks down the surface and provides adhesion for the topcoat.
When the primer has dried, you can move on to painting. I'd suggest using the White Knight Rust Guard Epoxy Enamel Spray Paint in a colour of your choosing.
To prevent the problem from returning quickly, it helps to maintain the area. After repainting, keep an eye on those ledges after heavy rain and wipe them down if moisture sits there for long periods.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
