So far out of the 8 hawkers I've had 7 good to go after some time (24 hours per) on a smart charger. Two of those were at .4V so it's possible to save them but they may be at reduced capacity to some extent. Here's some info off the manufacturer's site to charge them, I followed it. Don't go over 15V or you will cook them. Also here's the charger I used:
Schumacher SC-10030A SpeedCharge -
1-6. DEEP DISCHARGE RECOVERY PROCEDURE
1. If a battery is heavily discharged, usually due to a low current drain, it is said to be in a deeply
discharged state. This becomes apparent when the OCV is less than 10 volts.
2. Constant voltage charge, using a standard charger set at 14.25v +/-.25v. Restrict the
maximum available current to 5 amps, initially the battery may appear as if it is not accepting
charge, (this is due to the sulphation barrier that builds up within a deep discharged battery)
but after approximately 20 minutes the current should begin to flow. Doc Ref : 2602-0052 Rev 00
This is the safest method of recovery but can take 36 hours or more to fully recover the
battery.
3. Perform a Capacity test in accordance with para 1-7. A battery that provides less than 300
Minutes (100%) duration but in excess of 240 Minutes (80%) is still suitable for use and may
benefit from additional charging.
4. If the discharge duration is less than 240 Minutes, it may be assumed that the battery is unlikely
to be recovered and it should be rejected.