hey, you pay for cheap stuff, you get cheap stuff. basically you get what you paid for. so seriously this rust stuff, how many times have i been tellin you to get new body panels welded on, if you don't got the money i onknow what to tell ya, but yea you can fix it now, but what about down the line a year from now, when your nice new paint job is all jacked up cuz that whole quarter panel collapsed in.
as for salt in the engine bay, i'd use degreaser, hopefully that'll get it out, and i believe in the articles section it tells you how to clean your engine bay step by step, maybe that will help.
and turning rust into black primer? you got to be kiddin me right...no way is rust elimated by just a spray, if something is strong enough to remove rust by a spray, don't you think that stuff would mess up the sheet metal of the car as well....honestly cuz i'd save up and do it the right way, take it to a shop and have em weld you a new quarter panel and fix that roof rust as well, if now yea cheap fix it, but then sell it (hate to say) but eh you do what fits your needs/wants and your budget. nukka sell the car.....lol