Depends totally on the existing surface as to the proper plan of attack.
For example, the pic of the hinge shows what typically happens over the years and multiple paint jobs. Small, odd shaped pieces that are really hard to sand usually don't get prepped(sanded) well so they eventually fail(peel). The flat, easy to sand areas are easy so they have a better chance at proper sanding so they are less prone to failure. Guessing that the cowl, frame, etc is tight (I.E. not peeling), I might tape off and sand blast to bare steel just the hinge, and sand the "good" areas normally.
You may find most everything is pretty good shape but that fender and door that got replaced was painted after a hungover private used gasoline for a final wipe down just before painting and only those pieces need totally stripped.
You may find that between the military and the civilian owners, the truck has 12 different paint jobs over the years and NONE was done correctly (how far down do we dig now and which layer/s is failing?)
If the majority of the vehicle is "tight" but just has some normal chips then one could sand down and feather edge those areas as you go. The whole idea is to not have paint falling off, so if any areas look shady, spend the time needed to assure a satisfactory bond. Sometimes you have so many issues that the only way to be totally sure is to sandblast everything down to bare steel and that is lots of work,time,and money.
Hope this helps, sorry if it confuses or frustrates. Painting can be easy but the bottom line is proper prep of the surface.
I remember years ago a guy asked me if I would teach him how to paint--I said sure thing--ya got a couple of spare years? He walked off with that deer in the headlight look.
Jim