Another thing is, tank trucks are constructed of various materials depending on what products they are hauling. Aluminum, Carbon Steel & Stainless Steel may all look like
the Alclad aluminum of our Airstreams, but they are not the same.
Alclad is a trademark of Alcoa used to describe a corrosion resistant aluminium sheet formed from high-purity aluminium surface layers metallurgically bonded to high strength aluminium alloy core material. Alclad is a heat-treated aluminum, copper, manganese, magnesium alloy that has the corrosion resistance of pure metal at the surface and the strength of the strong alloy underneath. It's the same material that they use on airplanes - but not usually tanker trucks.
Another analogy is to compare a piece of "gold filled" jewelry to a piece of 14 or 18kt gold jewelry. Gold filled is a thin coating of gold layed over a base material (sometimes silver, sometimes a something else) where as 14kt gold is the same material through and through. The difference between 14, 18 or 24kt gold are the alloys - there is more pure gold (24kt) in the 18 and the 14 even though they are all "solid" rather than "gold filled".
Tanker trucks are not the "layered" sheet - they are the same alloy material through & through, so sanding it isn't a problem. If you sand through the thin layer of corrosion resistant aluminum in Alclad, it's still aluminum below but it is not as pure - it's a different alloy and so it will corrode (oxidize) at a different rate (faster) than the areas that still retain that top layer. Unfortunately, you can't see it happening as you are working an area, it'll show up as dark clouds a couple of days (weeks?) later - by then it's too late.
We too very lightly
sanded some of the front road rash effected panels before polishing, but it was with 1000 grit wet-dry color coat paper with good results. The reason we did was to knock down the craters or pockmarks of the rock dings/road rash before polishing so the craters could be smoothed out - not to remove oxidation. Every little pebble that hits the front panel, creates a divot that when you go to polish, catches the buffing pad and compound.