There are Rust remover cleaners available in market. Find the good one. But use mask and gloves while doing it as diy because such cleaners has very low ph (means they are acidic).
Once all surface is rubbed clean with brush & MPC, give it a good wipe and let it dry.
Meanwhile get the white permanent marker from stationary shop (or paint marker) and use it here.
I believe it would be the most economical solution of it but it would be temporarily solution only.
The ideal & long term solution is to get it fixed from dentor/painter properly who will use sand papers to remove the rust & paint also and then re-paint the surface area.
