6' X 2 1/2' X 1"
Um. 1" deep? You may mean 1 foot?
Current flow is a function of
1. surface area - both part AND anode (cathode?) Should be equal or greater on sacrificial part. A metal tub would work GREAT but will have a short life.
2. Line of sight - closer is better but adverse to even electrolysis. You may have to do one side and then the other.
3. Amount of ions in the water - more acid or (usual) base.
Check your connections. A good connection to your piece is essential. A large current with a small voltage differential (most of the dV will be in the bath) makes eliminating resistance between the wires and (particularly) the part or anode/cathode connection essential.
I use lye (yes, that Lye) but in rather weak solution. I can put my hands in the bath with little issue EXCEPT I can feel any cuts in my hands - and my hands come out SPARKLY CLEAN afterwards - well almost to the point of being raw after moving parts in and out over a day.
If you've tried all else like positioning the anode/cathode closer, increasing voltage modestly, checking your connections, try making a stronger "brew." And resort to gloves if you find yourself at all uncomfortable with putting your hands in the solution. It should not hurt.
And don't stick yer mits in the solution when the current is on. Micro-amperes at low voltage can result in death if condition/path is correct.
http://myweb.tiscali.co.uk/andyspatch/rust.htm
Reading above I've refreshed my memory. Object to be cleaned/derusted is CATHODE and is (-). Sacrificial item is ANODE and is (+) If you hook up backwards you may get the deposits you mention. My anode tends to get a kind of a greenish whitish deposit which looks oddly like wet fungus on a rock. You can brush this off and it improves current flow.
Joe K