We have our own in-house paint shop and full-time staff to run it. It's exclusively for VW camper vans (and the occasional Beetle or Karman Ghia) we don't do blow-overs or knock dents out of Taxi's. We just paint campers to a very high standard. We always remove all the windows, doors, bumpers etc and use high quality paints and materials.
The main problem with all of these 30 odd year old vehicles is rust. There are 2 main types that affect the longevity of any paint job:
There is real bad stuff that perforates the metal leaving holes in the painted surface. The only way to remove it is to cut it out and weld in fresh steel. Great care must be taken to not store up more problems for later and a skilled welder will use a butt welding process that leaves the finished surface as smooth as possible, minimising the use of filler to finish off the surface.
And the (apparently) not so bad stuff that sits on top of the metal and bubbles up through the surface. This is the rust that keeps coming back. Body-shops try sanding it off, grinding through it, painting chemicals over it or just hoping it will go away. It rarely does no matter how hard one tries. We have recently invested in a brand new media blasting machine from the market leaders, Hodge-Clemco. Its a full-size proper piece of kit powered by a the kind of compressor used on a road drill. This machine uses a special grit to remove everything from the surface right down to naked steel. Then we immediately apply an etch coating before surface oxidation takes place.