[acidrain]

3.5 amps per sq. ft. is too low... I wouldn't go any lower than a 4.5A/sq.ft. CD. 6 is perfect for home ano in my opinion.
18% by volume (battery acid:distilled water)? That is still a bit low... you want 25% (1:3 by volume). Making the acid stronger will make it easier for the current to flow... for the same amount of amps, the volts will be lower, therefor, if you are limited to 12v, the amps will be higher (higher current density). Same with acid bath temperature. Lower temp is harder for current to pass. You should try to get your acid bath to 68F-70F (no higher than 70F, or desolution will occur).
If your temp is low, try 68F first (use a digital thermometer... ~$10 at Harbor Freight).
If that still doesn't get you to at least 4.5 CD, increase your acid to 25%.
Remember, only change one thing at a time when doing controlled experiments. I would definately start with the temp... it ALWAYS has to be 68F-70F.
Oh, and you didn't say... are you using Caswell's dye?