Where your going to metal hit it with 80 and then feather edge it with 180. I guessing the paint elsewhere is pretty beat, so I'd look at going at it with 180 may be 80 depending on things. For example, when I did mine, I started with 80 to take off the rattle can and get down to original primer in a quick fashion. After some observation, the primer was checked or crazed if you will from the sun....out came the 36 at that point as I knew the end of he line for me was bare naked steel.
When you get it down to the point you are happy with the looks and say you have some bare metal spots...prime them up first with 3 coats of primer covering a progressively larger area with each coat. Then hand sand/block it out with a little light sanding with 180 followed with a light sanding with 240. Next prime the whole pig with 3 coats of primer and then sand with 240 on the DA. Using a soft pad on the DA is invaluable ...keeps you from burning through the edges. Use a 3M scratch in those tough spots. After this, blow it down real good, clean it with prep solv, tack it off and then paint on the sealer. Then tack after dry to touch and put on the color. Depending on the paint type your using it will vary from here on out.