Hi @Chicksy,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Assuming the lintel is still structurally sound, grinding back and treating the rust is simple enough. If you are confident that the lintel is still in good shape, the process will involve grinding back the rust, treating it and painting it. If you are not confident it is still structurally sound, I would suggest you have an engineer assess it to ensure it is still up to the job.
When you are sure it is good to go, after you have ground back the worst of the rust, apply Rust Guard Rust Converter to the surface of the lintel and allow its chemical reaction to take place. The rust will start to turn black as the chemical reaction takes place.
Once treated with rust converter, apply a metal primer such as this White Knight Rust Guard Medium Grey Metal Primer Paint before applying your topcoat.
Let me know if you have any further questions.
Jacob
