After carefully removing all of the surface rust, debris and stuck tar paper and sound deadening material, we painted the rear area with two coats of primer, one a deep etching sealer and the other a rust preventative primer (both had from our local home supply store). If you put your thumb over the picture to cover the huge holes in the tray, it looks pretty clean, right? There's still work to be done back there, as this is only a temporary fix.