If you have it bubbling then you are up to gassing volts, check it with your voltmeter on charge to see what you get.

6.8 v off charge is probably reasonable and if it holds to 6.4v when off charge for 12 hours then you have got it back up.

You can reduce the charge rate once it is gassing and leave it gassing gently for a while, it is not a good idea to hold the 10A after it has reached gassing volts, but with that size battery it will not be disasterous for a few hours. The aim is to keep it at gassing voltage until the SG on the hydrometer stops rising. If you only have the thing with floating balls then it will not be much of a guide.

If that was the worst one then you are likely to be able to recover the others.

When they are all back up you will have to find out why they went flat in the first place. Your dump load was set far too low, but you claim they never reached 13v.

One battery bad, therefore the string is not fully charged -- so do the whole string.
You can place each battery with the 6 volts charger and bubble each one of them as indicated if the wind mill can not do it well.
Set the dump as indicated before.

The batteries If you leave them for a while to hard charging that is the equivalent to deep equalization that is what you need to do to salvage the battery -- the 6 volts hit with 10 amps for about 24 hours causing bubbling --an indication that you have been able to fully charge the battery as the battery is with a possible some sulphation -- discharging the battery and another charging cycle will assist in further removing the sulphation.

I gave you 2.46 volts/cell so for you to calculate the final voltage for a 6, or 12 volts or even 24 volts string.

So for 6 volts is 3 cells * 2.46 = 7.38
for 12 volts is 6 * 2.46 = 14.76 volts
for 24 volts is 12 * 2.46 = 29.52

When the battery has sulphation one needs to hit the battery HARD and make it BUBBLE and keep it for 2 or 3 hours -- this for the current to go deeply in the plates and do the conversion and remove the sulphation -- discharge to 1.75 volts /cell and repeat the charging cycle.

Measure the batteries during charging, Voltage and hygro to observe how one battery is charging against the others in the string, the weak one will have lower voltage that the good ones -- the very new may have the lowest voltage -- notice which is new and which is old and low voltage.

I do not have the time to really explain such battery behavior.

The 6 volts battery that reads 6.8 volts charged and disconnected is telling you that is fully charged and possible "cured" from sulphation, a discharge cycle may tell you if you have full amp-hours capacity --so do it -- and charge again.

If one string is constantly up/down in charge/discharge cycles and the discharge is lower than 1.84 v/cell, then the string needs periodic and often deep equalization - for this reason is always necessary to have a good hygrometer and take reading constantly if you want the batteries to last.

I learned this from my family electrical equipment that had large open lead acid batteries and Edison NiFe batteries -- that with the proper maintenance were maintained for many years -- the lead acid were abandoned around 10 years ago ( bought in the early 1900) and the NiFe still alive and well -- now a bit hard to find Lithium Oxide for the yearly cycle maintenance periods -- KOH oxidizes if exposed to air - so it needs periodic replacement -- it is covered with film of mineral oil to reduce the exposure.

The gray ( gray toward whitish) films in some batteries types is a sulphation product.

An electronic desulfator will assist in the "cure" but if the batteries are not properly maintained they will not get good desulphation "cures".

I need to quit and go to a meeting.

What does the .1C mean?