We used to do the epson salts treatment. We dumped and rinsed out the battery, then refilled with just plain water and some epson salts, but I don't remember how much. As I recall, though, those batteries did OK during the summer, but couldn't make it through the next winter.
I've also heard that you should never add acid to a battery, but don't know what the problem or danger is supposed to be. I have sometimes wondered if you ran a battery all the way down, on each cell separately like you could on those old batteries where there was an external lug connecting each cell, then reverse charge it for awhile, to clean the plates, then run it back down again and recharge the correct way, if that would work, but it's all a whole lot of trouble for an old battery.