At work we just replaced a horizontal Emglo that was used to maintain the air pressure on a dry sprinkler system. Less than 10 years old, it had a pinhole leak in the bottom midsection. The paint had blister about 3" in diameter, but hadn't cracked or split.
This unit was manually blown down once a month as part of a PM. No water to speak of was ever blown out, just air. It cycled very little, as the system is pretty tight. Oil changed as spec, never had to add oil between changes.
I hauled it home and cut up the tank to make a fire pit. I started by taking a ball peen hammer to the blistered area, listening to the sound difference between a sound spot and the bad spot. Very quickly, a 1/8" hole opened up, and another about 3" away. I used a pocket scewdriver and probed and wiggled at the holes and they didn't get much bigger.
Using a zip wheel I cut it open lengthwise. It had a good inch of oily sludge in the bottom, drain was clear. In the midsection at the bottom there was an area about 6" diameter that was pitted pretty good. No other areas showed signs of trouble.
If anybody's interested, I can post some pics and more details.