OK, I've decided to do a respray of this case:
...and do a decent job of it this time.
I've attacked one side panel with a Black & Decker sander today:
Very satisfying! It's the hardest panel to do thanks to the grill at the bottom - I couldn't spend too long on that with the sander as it catches, so it'll be a manual job that bit. Still, the rest of the case is "normal" so that'll be easier.
A window is gonna replace that ghastly 120mm hole (and a new 120mm hole is going at the back of the case instead of the 80mm one that exists currently.)
Just thought I'd share this with you.