Here is some info about my process....
Prep work, by far, is the most important process..
If you have any scratches in the metal, they will be shiny scratches when your done..
There are some fillers available, I have tried a few but they all have their own quirks..
Candy coating (2 stage process) is much more difficult... the cheaper gun setups usually do not have enough adjustments to get a even coat on the second stage and they waste quite a bit of powder.
Doing multiple colors requires a partial bake of each color before going to the next color or you will get cross contamination.
Cast parts have a tendency to outgas... rough castings are the worst, there are some powders that are outgas forgiving, but not in many colors.