when repspraying areas in need of a respray, do the car in sections, sure its an arse of a thing to do, but it will make the whole job look alrite.
doing small patch work on a car thats getting a bit old will never look right as most paints match the colour of the factory, but since time has aged the paint and it will not be the same anymore. so if theres a patch (large due to fading) on the roof, id suggest cut back the whole roof and all 4 pillars (front and rear)mask up down to the bottom of the window and respray that section..
u dont need to cut back to bare metal only primer, and spray putty comes on handy too to smooth out undulations. the more time u spend on the body of the car the better it will turn out. currently my car is basically in primer after being cut back and all straightend, hehe it looks like shit (more like a grey battle ship) but it will get resprayed in a week or so at work.
quick guidline (pretty rough) for respray -no dents on panel
1. cut back paint to primer with random orbital sander using 100 grit sheet
2. prepwash area cut back and hand sand any minor undualtions in paint work down to primer
3. spray area with super etch primer (2-3 coats) and lightly sand with 300grit paper, prepwash
4. spray area with primer surfacer (2-3 coats) and lightly sand with 300 grit paper (very lightly) then should bring up the high and low spots of paintwork, prepwash
5. low spots can be gone with spray putty and lightly sanding area, use a low brightness lamp shining over the paint work to find imperfections
6. apply another 1-2 coats of primer surfacer and lightly sand again. pref 600 grit paper
7. guide coat. this is very important to find out how straight the panel is. this should only be applied as a mist (very very fine) and let to dry. sand back area with 600 grit paper (lightly) pref matt black
8. car is ready for top coat and its up to the user of the desired finish, get hand of spray gun preferably! and paint mixed up to suit.
9. cut back with light cutting compound about 4 days after top coat applied.
then polish up.
always use compatible primers with acrylic, and compatible primers with enamel.
heeh i hold no responsibility for this info. this is just a basic outline and its up to you how much work u want to do.