You can see flash rust on the sides, probably from the hydro test. But the bottom is way past flash rusted.
You can also see some of the stains from the water on top of the existing rust in the bottom of the tank, indicating it was there prior to the water causing the flash rust on the sides of the tank.
The rust on the bottom is thick enough in places to obscure the tank and I'm amazed the RIN passed this tank in the first place - it should not have passed the VIP portion of the re-qualification in that condition.
Tanks are often tumbled before the hydro test, to ensure that it will pass the VIP portion of the re-qualification which includes by the hydro test and visual inspection.
Tumbling will remove the rust, and once the rust is removed, you inspect the surface for pits. That's the reason the obscuring rust HAS to be removed.
If the surface is pitted, and the pits are deeper than allowed the tank will be condemned, regardless of whether it passes a hydro test.
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To properly tumble the tank you need to fill it more than half full of media, so that the media is able to reach the parts of the tank in the horizontal axis of the tank. The center of the bottom of the tank also sees a bit less cutting action, so you need to tumble long enough to get that part thoroughly cleaned. It may require an over night tumble, or it may require 2-3 days of tumbling, depending on the media used, whether they dry or wet tumble and depending on whether they use a cleaning agent with some phosphoric acid in it or not.
