You can buy a nylon scourer brush for a drill that are called graffiti removers. They are awesome for gentle removal of light rust. You could also try using a flap disk for your grinder. They are basically about 60 grit sandpaper for the grinder. A bit more heavy duty, though.

And the rust converter stuff, which i think is basicaly sulphuric acid is great. It stops rust in its tracks, but you are still better to remove all the rust first if you can.

The fish oil stuff worked for me in my old pootrol, but it was messy and needs regular aplication on external panels to maintain protection. Its a good cheap method of reducing rust damage, but its not very permanent.

If this panel is going to sit in the shed, then soaking it in fish oil wont do it any harm and you can just brush it on.

cheap, well sulphuric aicd for the rust
and used engine oil to protect the panel for storgae. Not pretty, but cheap was the critera.
If you actually want to put it on the car, then the fish oil is the next best thing, or CRC. that works alright.