I may have the perfect test-case tank. A 2003 tank, never hydrod, VIP'd once in 2004. The guy claimed it has been in storage for as long as he knew, and had no idea about the tank's history. Apparently he was a military diver, he knew how to dive, but apparently didn't know much about equipment nor dive recreationally. At $25, worth the risk.
The inside of the tank was clean. I stuck a flashlight inside, and threads look good, and tank just looks like bare metal.
The outside is another story. Valve had a medium-high amount of corrosion on it's exterior, but was yoke meaning it's being replaced anyway. The paint was a pretty blue color, however it's bubbling with white powdery stuff underneath (presumably aluminum corrosion). In the areas I manually sanded so far, the metal underneath ls smooth and no signs pitting. I'll finish either when I sand-blast it, or a warmer day when it's more convenient to use some paint-stripper outdoors.
In other words, the paint needs to be stripped anyway, and a protective-coating would be nice. I can imagine a scenario where a dive-shop may notice the bubbling and refuse a fill or fail-VIP. It needs hydro as well, so might as well paint before hydro.
My plan for dealing with dive-shops is (1) start with a single tank, and see if local dive-shops give me trouble. (2) if I run into trouble, have photos of the stripped tank easy to access, along with showing the paint is air-cure (3) If all goes well, it might be fun to even sell/swap the painted tanks with other locals. Not as a business, but rather a distraction and practice, and something to do as I slowly upgrade to newer tanks and/or more Steels.