What I believe actually works to desulfate batteries is to dump the acid out, replace with distilled water, and float charge the battery at 14 volts for a couple months.
The reason this works is because the low concentration of acid, 1.05 or less, produces zero "volts" in a healthy battery. as such the lead sulphate crystals "see" 2 volts across them.
where as with the nominal 1.2 sg acid, the natural voltage of the battery is 12.6 or so, and a 14 volt float charge is only pushing .2 volts per cell into the battery.
in one case, i got an additional 2 years functional use, out of a 10 yr old car battery that had declined in capacity to just 3 amp hours. after 2 months of float charging after dumping the electrolyte out, the acid concentration rose to an unbelievable 1.2, the amp hour capacity was around 20, and i put it back in my truck.
I don't have a good understanding of where the acid came because i may still have the 3 liters of 1.25sg acid i dumped out. in any case, there is about 4-5 times as much "active material" in a lead acid battery than there are amp hours.