Hi @emmab1986,
The rust coming through the render is because there is a metal bead running along that corner that has obviously gotten wet and, over time, has rusted. There are plastic beads, that can be used instead which would not be susceptible to rust.
The trade you would need to speak to is a renderer. They will be able to scrape back the existing render, remove and replace the bead and then redo the render.
Let me know if you have any further questions.
Jacob
