plastic paint???

Should I blast it down to clean meatal, or just sand out the scratches, gouges etc, and go over the top???

Huh ? You wouldn't sand blast the vinyl skin off it. If the raw steel inner side is rusty I will use Rustoleums Rust Reforemer primer and nothing more. There are ways to repair small gouges in the vinyl skin but its a little time consuming. I put most of that time/effort in the seat backs where it matters most. Small scars in the skirts are hardly visible after a fresh coat of the SEMs.