The problem with over discharged lead acid batteries is the build up of lead sulfide, By pushing a high current through the cells you can blast the sulfide off the plates but the sulfur that went into making the sulfide came from somewhere and that somewhere is from the acid. What I have done in the past with traction batteries is replace the acid then charge the cells individually until they "milk" the electrolyte goes white with bubbles, then remove the acid wash out the cell with deionized water to remove the sludge and the replace the acid with fresh. You cannot do this with gel,absorbed or sealed units, it is also not worth the cost or effort on small size batteries and the money spent on buying a battery charger to try and fix the problem is better spent on a new battery.
You can blast AGM batteries as well. Most AGM chargers does that once pr. month or thereabouts. Normally you say AGM batteries can be charged at their AH rating divided by 8 (so 100 Ah battery = 12 amp charge) - Lifeline batteries are different - then can take Ah/4 - so 24 Amp for same battery max.
A "normal" 14.6 volt charge from a "standard" Led Acid "non smart"charger is actually quite equal to the "Blast charge"(Constant Voltage) you would get from smart charger once pr. month or on extreme low depleted batteries. So just use an old fashioned car charger to see if you can kick it back into life. But if battery "measured" voltage after charge does not change or drops quick - one or more cells are dead.
Normal 3 stage charge is:
1. Charge with Cell Voltage of 2.35 (times number of cells)
2. When battery reach the charge voltage (2.35v x number of cells) switch to Constant Current at Charge Volt = 2.25-2.27 x number of cells) for 3-4 hours
3. Switch to 13.6 volt trickle charge
And then "smart chargers" have a "if battery was depleted a lot - or once pr. month"- continue "step one charge" for an extra 3-4 hours.
I would try to charge with a normal old car charger for 30 min - > 1 hour - and then check if battery voltage has increased (disconnect battery from charger and let it rest for 3-10 minutes before measuring) - if Voltage has increased - continue... if not - dead cells... and no way forward.