We call this the "hydro trap", where you take a tank in for a visual, and the ship insists on tumbling it, then says it has to be hydroed, and your $10 visual has become an $80 major event.
It just isn't true. The amount of metal that tumbling removes is insignificant. Any shop that insists on rehydrotesting after tumbling is either after your money or doesn't know what they are doing. One might argue that a tank with line corrosion or deep pitting, which barely passes visual, might have enough metal removed to require rehydroing, but such tanks are rare, and usually better discarded. The DOT regulations, when they list situations in which immediate rehydrotesting are necessary, do not give tumbling as one of them.
We always recommend that anyone with multiple steel tanks learns to remove the valve and do a quick pre-visual before they take a tank in for a visual. If there is minor rusting then it is often easier and cheaper to remove it yourself, using an easily made stainless steel cable whip, before you bring it in for visual, so as to avoid the tumbling-hydro arguement altogether .
It's always a good idea to find out before you bring a steel in for visual what the shop's policy is on hydroing after tumbling, and whether they will automatically tumble and hydro any tank that fails visual due to rust, or whether they will call you first.
If one has taken a tank for visual, and the shop tries to insist on a tumble and hydro, we usually suggest getting the tanks back from them by any means, then either cleaning them yourself of taking them to a shop that doens't require tumbling after hydro and having them cleaned and inspected there.
JBRES1:
After a tumble , you need to re hydro to make sure the tank is ok.
Tumbleing media can remove material from the inside of the tank other than the rust.
Jim Breslin