I'm currently de-rusting the tank for my K6 project, it's a beautiful dent free OEM Valley Green (?) K4 tank but sadly, inside there's more rust than steel.
I used the tank cleaner from my POR15 kit but that hardly put a dent in the huge amount of rust inside it, so I thought "nothing ventured, nothing gained", so I used an old engine mount bolt (the long one) as the anode, filled my tank with a washing soda mix, and hooked up my crappy little 2.5 amp battery charger.
Now being that most articles here and on the interweb talk about using 8+ amp chargers (in contradiction to the linked article above, surprisingly) I wasn't expecting much, so I was pleasantly surprised with the large amount of rust I witnessed hanging off the anode only a couple of hours later!
Don't get me wrong, I'm only just noticing clean metal inside the tank now (on the anode side of the "hump" mostly) after 5 days of electrolysys so I might well need a bigger charger, (actually I bought a 16 amp charger on EBay but it's one of those stupid automatic "smart" chargers and it won't work on my electrolysys rig at all, so I wasted 105 bucks, bugger it...........) but regardless, it's producing a better result than the acid did. Cheers, Terry.