How much sanding?

Hey fellas, quick body work question
Im am doing rust removal from a fender i picked up from a junkyard, and i don't know excatly how much to sand. I've been studying up on bodywork and i want to start doing more of my own, so the stepside fender has been good practice for me. The new fender i got has little tiny chips everywhere and i am removing the rust on each one with a DA. For each rust spot, i am sanding about a square inch of paint off to the sheet metal. Since i have so many little spots down to the metal, should i just sand the whole fender down to sheet metal, or should i just sand all the rest of the paint down to where it is extremely smooth to the touch and sight. i don't want a an uneven painted fender when im done. The hard thing is, i am an perfectionist, and since i havent done a paint job before, i don't know excatly how perfect the body work has to be for it to turn out good without any flaws.

I think Spraytech might know more but I have found the when you take it to bare metal you get a better adhesion for the primer/paint. Try mediablasting the fender to bare metal before you spray the etching primer and then the finish coat.......worked for me and job came out beatifully.:o

I will second what trog has mentioned, but add to it .

Beings you said you had a DA, I would just set it on grind and put 80 grit sand paper on it and just rip it down to bare steel, this will take off what little rust there is in the rock chips, then put 180 grit on your DA on the feather sand the metal to smooth the 80 grit grinder scratches out .
Its not a good thing just to sand a ton of little spots through the paint as it will most likely show little rings( we painters call *assholes* even through the paint job as they are real tough to put enough primer on it and block it down.

You are suppose to feather edge the paint back 3 " for every layer of paint that is on the panel.( say it has 3 layers , the bottom layer you should sand till you see just 3 " of that , the on top of that layer 3" back , then the last 3" back from that.) So by the time you do this to every spot you will have it down to bare metal any way if you have done this with every chip !
Then try and get some primer on that bare metal as it will surface rust in a few hours time.

OH and prep work is 90% of a fine paint job , if PREP work is not right ,the Paint will only magnify whats underneath it !

SprayTech