to be fair, if you know what your doing and have good skill DIY can be done.
But on a large flat panel like the 350z rear quarter, its a tough job!
I did my scooby rear arch couple of years ago, as i was selling it and the arch was rusted, i got it all clean, meshed, fibreglassed, fillered smooth. then i blended in the paintwork stepping it into the OE paint. Machined it back afterwards, and considering where i did it (front garden) it came out bloddy well. but that was just a thin rear arch, and a M reg impreza..