On Monday, September 3, 2018 at 6:27:40 AM UTC-4, unk wrote:
I've done some electrolytic derusting of small things with a 10 amp
battery charger. Works great, biggest was a Vespa frame, took some days
as I recall.

Now I'm thinking of a car frame. I don't mind leaving it for a while, but
is that charger going to do anything at all or is there a sort of
threshold current/area ratio?

There are thresholds, but explaining it is not simple:

"The current density used in electrolytic reduction is expressed as the number 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 electrolytic cleaning (Plenderleith 1956:195; Plenderleith and
Torraca 1968:242; Plenderleith and Werner 1971:198; Pearson 1972a:12; Townsend 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 scientifically. I'll give some references below, if you really want to get on top of the subject.

First, I'm going to second the recommendations of others and suggest sandblasting. I watched my father's boat trailer being sandblasted around 30 years ago; it was quick, thorough, and fairly cheap. It started to rust again in about an hour g, 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 objects using different methods, keep in mind that the archaeological restorers 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 English cast iron, used a setup of less than one ampere for a whole cannon. However, the process takes five to seven years...

Don't get discouraged. They were not concerned with removing rust. They were 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, anyway, or you may find lots of pockets *under* the paint, where it got nicked and the rust spread under the paint. Electrolytic rust removal, in my experience, won't do much about them unless you get the paint off first. So you'd might as well go with sandblasting to begin with.

If you really want to understand electrolytic rust removal, these are my two favorite references. I follow the low-amperage route except when I'm using the "wand" method, with a carbon electrode. I use a piece of an old EDM electrode for that. You won't find much literature about it. It's good for thin 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 when you want to get into the electrolytic process:

"Rust Removal Using Electrolysis" by Andrew Westcott:

https://www.qsl.net/2e0waw/rust.htm


"Methods for Conserving Archaeological Material from Underwater Sites" by Donny L. Hamilton, Conservation Research Laboratory, Center for Maritime Archaeology and Conservation, Texas A&M University

http://nautarch.tamu.edu/CRL/conserv...tionManual.pdf

The latter is the source of the quote I pasted above.

Good luck!

--
Ed Huntress