Random Orbit Sander
For the flat areas if you have one use a random orbit sander with 60 to 80 grit sanding discs. Does a real nice job of removing the rust and scale. If you need to borrow one, let me know.
As mentioned before do it outside if you can. Use compressed air after sanding followed by a vacuum and then a tack cloth. I always found a couple of light coats with a scuff sand in between worked better than 1 heavy coat.
This is excellent advise for painting a stove. It is what I did to my wood stove insert. Don't press too hard with the 80grit or you will have swirl marks on the steel.
As for the Stove polish, I have never used it. I looked into it, and the directions said it was for use on Cast Iron stoves. I do not think it is designed for Plate Steel.
My insert is Steel, so I decied to use stove paint. I bought the paint from Home Depot, Rust-oleum high heat (in a spray can) resists heat up to 1200 degrees. It was $12. Insert looks like new. It was smelly for the first few fires until the paint set.