All of that sounds fascinating, but for a one off project like a car frame wouldn't a sand blaster and a big bag of glass beads be the answer? Some time back I converted a boat trailer into a flatbed utility trailer. We did the whole thing, and blew the slag off the welds we couldn't reach with a hammer or a wire wheel with a hand held self contained sand blasting gun and a bag of playground sand.
There are thresholds, but explaining it is not simple:
"The current density used in electrolytic reduction is expressed as the num ber of amperes per unit of artifact surface area that is introduced into the electrolytic cell by an external DC power supply, such as one ampere per square centimeter (1 amp/cm2). Current density ranging from 0.001 to 1 amp/cm2 have been proposed for use in elect rolytic cleaning (Plenderleith 1956:195; Plenderleith and Torraca 1968:242; Plenderleith and Werner 1971:198; Pearson 1972a:12; Towns end 1972:252), but guides to the application of specific current densities are seldom given."
I paste that just to show that the subject has been dealt with scientifical ly. I'll give some references below, if you really want to get on top of th e subject.
First, I'm going to second the recommendations of others and suggest sandbl asting. I watched my father's boat trailer being sandblasted around 30 year s ago; it was quick, thorough, and fairly cheap. It started to rust again i n about an hour , but that was in salt air on the shore of Barnegat Bay. If you live near salt water, you should be able to find these services.
But if you want to go with electrolysis, which I do all the time on small o bjects using different methods, keep in mind that the archaeological restor ers who used the method to restore the 18th-century 6-pounder cannons from Blackbeard's "Queen Anne's Revenge," which weighed one ton each and were En glish cast iron, used a setup of less than one ampere for a whole cannon. H owever, the process takes five to seven years...
Don't get discouraged. They were not concerned with removing rust. They wer e reducing the levels of salt that had penetrated the iron's pores. That's a whole different problem.
Speaking of problems, you're going to have to strip any remaining paint, an yway, or you may find lots of pockets *under* the paint, where it got nicke d and the rust spread under the paint. Electrolytic rust removal, in my exp erience, won't do much about them unless you get the paint off first. So yo u'd might as well go with sandblasting to begin with.
If you really want to understand electrolytic rust removal, these are my tw o favorite references. I follow the low-amperage route except when I'm usin g the "wand" method, with a carbon electrode. I use a piece of an old EDM e lectrode for that. You won't find much literature about it. It's good for t hin layers of rust on small objects. It would take a month or so with a car frame.
These are the references I like. They're worth downloading for some day whe n you want to get into the electrolytic process:
"Rust Removal Using Electrolysis" by Andrew Westcott:
"Methods for Conserving Archaeological Material from Underwater Sites" by D onny L. Hamilton, Conservation Research Laboratory, Center for Maritime Arc haeology and Conservation, Texas A&M University
The latter is the source of the quote I pasted above.