The bubbling isn't rust so much but electrolysis (the panels are aluminium) but it looks like crap doesn’t it? the plastic frame can be removed and replaced but why? the plastic hasn't rusted away and that’s not your problem, but slide them out don't pull on them when you take them out, you will brake the buts holing it in if you do, if you want to DIY it, treat it much the same as mild steel, but when you spray it you’ll have to use a etch primer on it so the undercoat and top coats (and varnish) wont all peel away, i’m no expert but that’s the way I understand it