what to do with this

[truckman32]

to start i'm not a body repair man but (i can do it) with some help from you all, i am a painter though not a pro but well you know lol,my ? is what do i do to get rid of i guess it's called pitting as you will see in the pictures i was sanding my 62 fleet bed tody and ran into this pitting what do i do to make this smooth,and after i make it smooth what next.i have painted some vehicles for people but the body work was allready done .so now that i'm restoreing this truck myself i have to learn bodywork since i can't afford to pay somebody else