In the April “Yowl” pp 361 Dave H. Frank raises the spectre of rust.
I have had great satisfaction with the following:
NOTE: All acids must be treated with great care but Orthophosphoric acid is fairly benign particularly in the diluted form. The obvious precautions should be taken, safety glasses, rubber gloves, avoid skin contact and don’t drink it. Do a Google search for more comprehensive information.
Make up a 15% mixture by volume of Orthophosphoric acid with distilled water. To this add a 5% solution of Tannic acid.
The Tannic acid crystals should first be dissolved in a small quantity of Acetone and then mixed with the dilute orthophosphoric acid.
The resultant watery mixture may be applied by any convenient means and allowed to dry. Smaller components, if convenient, may be dunked in the stuff and allowed to soak, the brew will only attack the rust so you will not finish up with a bucket of sludge. It will loosen thicker layers of rust, which may be removed by any convenient means. Deep rust may require several coats.
The result will be a protective layer that will discourage further rusting. I was informed that rust pits form a battery cell and that the tannic acid acts as an insulator thus further discouraging rust.
Once cleared of all loose stuff the surface makes a good key for painting.
I have to admit that I am no chemist; the above recipe was given to me by someone who was. All I really know is that it works! Many moons ago I had an old Ford Cortina estate fondly known to my sprogs as “Rust Bucket”, (not undeserved). There was a particular stone chip in the middle of the roof that I never got around to touching in but about twice a year I’d drip a drop of my mixture on the spot which would put a stop on the “red stuff” for another six months. It also worked a treat in the bilges of my narrowboat and was cheap enough to slosh around.