X la where got skill enuff bro..myself oso stil learning...just that I have d opportunity to do it on a frequent basis n practice..sometime oso 'hancus' :)...
D thing about roof rust (any rust for that matter) depends on LUCK bro...d process n kind of products do play a part(x point u go all d trouble grinding,anti rust etc but use low quality products...x get me wrong x saying d painter did so)..taking care of it oso plays a major part..
For me I did d similar process u mentioned (except I did it DIY at home hehe)..grinded,acid it,anti rust it,putty/fiber it,primer it n lastly spray it (of coz d last 2 process did it at workshop)...after spraying anti rust inside out I sprayed d primer..sanded it..let it settle 2 days then sprayed color..I did 3 layers just to b safe s well to protect from debris n heat..lacquer wise I used hi-grade from glasurit oso 3 layers (I find it more thick n 'rubbery'...geeez sounds funny x it??)...this in my opinion helped a lot to protect from d sun..park in covered garage..but like I mentioned LUCK...