Reviving old batteries?

I have revived many,many, batteries,all sizes, with a long-term,low amp, hi-volt, automatic charger. Sometimes takes several weeks to a month and longer. The trick is to drive the sulfate off the plates and convert it back to sulfuric acid.
Sometimes tho, the cells are shorted,open, busted or somehow corrupted, and the voltage never comes up. Sometimes the voltage comes up, but they won't pass the load test. Sometimes they pass and then fail in months to weeks down the road. But I won far more than I lost.