Jeff, your tank is already the same capacity as my compressor and this tank combined! :-D If I had that big of a tank (eg vertical style compressor) on my compressor, I likely wouldn't have been keen. I just felt like 70l was too small, even though it ran pretty well without the motor powered up for me. This will last 3x longer than before if powered down.
The other advantage is of course the portability, but I can't think of too many applications where a limited supply would be acceptable. Track day tyre pumping, maybe, but a 12v unit would be a lot more space/weight efficient.
As for the treatment, it was pretty grotty inside with fairly substantial flaking rust in the bottom. It's not pitted and it's mostly still there (near original thickness), so I'm not worried about it being weak.
No idea about the black sealant, original owner (before last) was the one that assembled it in this way.
Zero concerns about the 3 cylinder compressor. The same company makes a continuous rated version with the same pump and motor, just a clutch mechanism so the motor runs no-load when not needed, and is engaged when needed. The wear/tear is on start stop, and this will reduce on/off cycles by a factor of 3, again :-)
ehb, I realised that the thread on the elbow is the same, so I might ditch that and have the socket horizontal, and the filler hanging down on one side at an angle instead. I agree that it's quite long leverage wise, however it shouldn't have more on it than the hose pull, really. I'll be careful, I promise.
Re thread, 12.5 to 12.6 TPI, 51.8 OD. Anything like that aside from M52? Not sure. Doesn't matter as I'm going to MIG a bung in there with a bolt and copper washer for venting if desired. I can cut it out with a hole saw and clean up with a die grinder if desired later. Going to rinse, zinc coat, and lightly oil the interior before doing that, though. Speaking of which, today's mediocre efforts:
https://twitter.com/FredCookeNZ/status/ ... 6783630336
https://twitter.com/FredCookeNZ/status/ ... 9429529601
Also worth noting, the wheels aren't on parallel. One pair is true to the tank, the other is 5 or 10 degrees off. The only saving grace is the wheels are so loose on the shafts that it allows them to do what they need to to move fluidly :-D