I recently tried to do the same thing. I have 2 Panasonic 6V/10A SLA's that were used for a few years in a dead UPS. I also have one 6V/4A SLA took from a emergency lamp that stayed on a wall for 7 years and was not operating at all for a long time. This battery is a chinese counterfeit, it's written Pansaonic on it, using the same font, etc. Shameful.
I used a desulphator circuit I made using the schematic I found on the Homebrew site. All the batteries were showing 0.5/1.0/1.2 Volts and 120mA on average each. Pretty much dead. The small one was light as a feather comparing to a new one so I presumed it's dried (7 years on a wall can do that).
I took apart the sealings and the rubber caps underneath and added distilled water to all of them. No change in status. Then I put the two big ones in series along with the desulphator in parallel and left them to charge using a wall adapter (12V/1A). The charging current was at first 20mA. I left the whole thing like that for 5 days and I observed the charging current steadily increasing. After 5 days it got to 1 Amp/h for the two series batteries. Pressure was building inside and I sealed the tops using hot glue.
The small one got the same treatment with another desulphator (I made 2) and a 7.2 wall adapter. The same symptoms, charging current increasing along the 5 days but faster. And this got warm after the 2nd day. The voltage increased to 5V and the amps my meter showed me were about 2-4. I left it there for further treatment.
The two big ones started to charge a lot after 5 days. The voltage was 4.9 and 5.1, the amperage got way up to 8-9 Amps. I had hopes. But after 7 days, to my surprise these two suddenly showed 1.0V each and 10mA (!) They also got warm. I said, well, this is it, it's officially dead. And I was right. Further attempts failed. No change whatsoever. I believe they got shorted.
But the small one, despite its manufacturer is now living a happy life! The amps are all there, the voltage never seems to go lower than 6.1Volts (higher neither) and I consider it a success. Not 100% success, but close. Usable.
My conclusions? I don't know. It takes a lot of patience, a plan, a chart to write down the readings on a daily basis and a bucket full of luck. I guess it's all about luck or how those dead batteries were used before.