Here is the manifold after 1 1/2 days at 30 amps.
What I use was the "electrolysis" method of removing rust. You will need:
A plastic tub or bucket
A box of washing soda
A battery charger (plus battery cables if you're using two anodes)
A piece or two of scrap metal (sacrificial metal / anode)
If you have a sandblaster, it would be quicker, but messier, and will also remove some of the good, clean metal. This might be a problem if you are repairing sheet metal parts.
In a plastic tub or 5 gallon bucket add plain water. Add 1 tablespoon of "washing soda" (not baking soda) per gallon of water. Mix until dissolved.
Place your object into the bath and clamp the NEGATIVE lead from the charger to it.
Now place your anode (sacrificial metal) into the water, but don't let it touch the part you are cleaning. Attach the POSITIVE cable to the anode.
(Note, if you want to speed up the process, use one or more anodes, just use the jumper cables to connect the anodes together)
I set my charger at 30 amps, less than that will work, but may take longer. After about 1 1/2 days I pulled it out. Here are the pictures.