[Krazy Hawaiian]

#1, you NEVER want to sand all the paint off.... theres no reason to unless there's been a lifting problem or similar. reason is: it's a waste of time, you will more than likely make a mess of it by making waves that will show up in the shiney finish.

#2 You don't want to put alot of primer on. Especially cheap rattle can primer. It will cost you more in the end when the shop has trouble because the paint they spray will make the cheap stuff lift or blister. Use only a 2 part uerathane or epoxy base/filler coat compatable with the top coat you or the shop is going to use. Also building up too thick of a coat just makes it chip that much easier and the chips will be deep and harder to fix.

#3 Don't use the single component type of putty. Use the 2 part Bondo filler when at all possible. The cheap single part stuff will lift and cause problems with the newer paints the shops use. Your much less likely to have problems if you use a two part compound thats reccomended by or made for the paint the shop will use, Dont matter if its under primer, the solvents will penetrate and still lift it.

#4 Compatability, thats the main thing to consider. If you use a product that isn't compatable with what the shop sprays you will end up wasting alot of time and money.