Pick up the spray bottle and squirt the pad, then mist the entire surface of the panel. Take a deep breath and start sanding.
It is very important to be consistent. Sand in rows, overlapping each pass on the previous much like how you run a spray gun. Do not move the sander in random patterns. Your goal is to remove the same material thickness everywhere on the panel. Random sanding tends to remove a lot over here and not enough over there....too easy to get a thin spot.
Sheet aluminum over ribs means the panel is slightly wavy; high at the rivet lines and low in the middles. Most of your sanding should be in rows parallel to the rivet lines, although early in the cutting process you can run patterns across them without risk. Keep the pad flat and don't press; just let it float. You want the foam interface pad to conform to the surface. As you near the end of the cutting process, just ease the pad edge up next to the rivet lines rather than over them and keep the bulk of the cut in the middles.
You'll build up a nice froth of powdered paint and water. After a few passes (in a pattern!) stop, grab a wet rinse rag (remember the water bucket?), and wipe down the surface. Grab a dry towel and really wipe the surface nice and dry. Now you 'll be able to see exactly how deep you've cut. Here is the surface after the first round, uncut on the right for comparison. See the shiny speckles in the tube light's reflection? Needs more cutting.
Ok, so mist it again and cut some more. Stop from time to time, rinse and dry, then examine the surface again. Move your head around to walk the reflection across the panel and look at all of it. You might spot an area with no more speckles while the rest still needs more cutting. No problem, just mark it with some tape dots. It may be thin there or it may not, but there's no point in taking a chance. That spot has been cut enough.
Continue working the panel. Eventually you'll reach a point where you can still see just a little bit of shiny speckle, the very last vestige of the demon orange peel:
You're done.....move to the next panel. All that's left is the buffer, which we'll get to in a future installment. And yeah, we'll fix a run too....