My lovely little Datsun Sunny

Well, here's what I worked on today. (Lucky for some, I know). My lovely 1979 Datsun Sunny Saloon. (Is it a Saloon? I'm kinda guessing.) I sanded some of the little rust spots off, and went over the bad bit with a grinder, but as I look at the photos I can see there's still heaps of rust! And how much do I get rid of? And what do I bog and what do I weld? I took welding classes this year, but I need someone to show me how to weld panels... I'm sure it's different to welding two pieces of piping together on a nice little work bench... And if I don't get rid of ALL the rust, will it be a waste of time priming and painting it? Will it all just come back?? Aarghhhh... o who cares. It's fun.
I'd better say, the reason I don't know how to get at all the rust is that I can't get into the backs of some areas coz theyre in like, a boxed in area. Like a hollow area, so I can't look in and see how much rust is in there. (Just so I don't sound too blonde... I know I have to get rid of the rust on the outside!)