A few auto body questions

i got a couple of fenders off ebay from a new mexico dude.

sanding helps me find all the dings / dents (must be a 100 lol)

anyways, what i have found is the lower 1/3 of the fender has pits under the paint.

not real bad, but even being in the new mexico desert, the doglegs both behind and in front of the wheel well from the bottom up to about 1 foot from the bottom had rust pits under the primer. == i'm goin bare metal and getting the pits out

i'm not sure the cause of this... my guess is normal driving whenever little stones hit the paint, even though they dont take the paint off, they make a "loose spot" which allows condensation under the paint at the spot where the rock hits. that is my only explanation why those areas were pitted up.