Chris, get a long board file and sand it by hand with 80. Get the paint off and try to flatten out the waves. Dont bear down, but let the weight of the long board do the work.
Go over it till you feel confident that it's flat. Then what I like to use is Evercoat Metal Glaze Part#100416.
Wipe a coat of that over it, then sand it with the long board with 80, then finish it with 180.
You can go over that again with 220 if you want, it will help get out any extra sand scratches.
Again, dont bear down when sanding, but let the weight of the sander do the work.
Good luck!