Brush on some phosporic acid or Evaporust on the surface, then cover with a plastic sheet to keep from evaporating. Every few hours brush again and let it sit
over night. The rust will soon be dissolved.
This is close to what I would do. But first, it sounds like the rust encapsulator isn't what you want to use on this job?. Your discussion would be interesting.
Farm equipment is usually a combination of cast iron and steel bar stock, each with a different surface, so you'll need to treat each differently. For example, you can use sandpaper to remove rust on bar stock, but not on castings.
First physically remove as much rust as possible with powered wire wheels or similar and hand wire brushing. Maybe some sandpaper, wet sanding, or filing as needed. Then clean well with soap and water or mineral spirits or both. Apply Evapo-Rust and keep the metal continuously damp by covering with rags or paper towels followed by covering with plastic to retard evaporation. Re-apply as needed starting in the morning and continuing for the rest of the day as needed. Of course, immersion is much better because this isn't the optimum way to use Evapo-Rust. In any case, the metal must be continuously wet and re-wetted, because small amounts of Evapo-Rust will soon be chemically depleted in the presence of rust and must be replaced with new liquid.
If all rust isn't gone, start over, finding better ways to keep the metal continuously wetted with Evapo-Rust. Inspect carefully, because red rust will be turned to black rust by the Evapo-Rust. Probe dark deposits with a file or similar.
While still wet with Evapo-Rust (re-wet if needed), wipe off, apply mineral spirits with a paint brush, and wipe off again.
Apply phosphoric acid (I use Rust Cure) with a latex paint brush because the acid will eat up natural bristles. Before it dries, wpe off, apply mineral spirits with a paint brush, and wipe off again. Phosphoric acid isn't very corrosive to skin, but follow all precautions like rubber or vinyl gloves and don't breathe fumes. Don't allow phosphoric acid to get on anything galvanized or zinc plated because it is much more active on zinc than iron or steel.
In rust pitted steel (not castings) there's the special case of rust still present in the bottoms of the pits as evidenced by dark spots in the centers of the pits. Wet with phosphoric acid and go over the area with a powered wire wheel turning slowly. Repeat until all dark spots are gone and all the area is metal colored.
The cheapest way I've found to get Evapo-Rust without paying shipping is to order 5-gallon buckets delivered to a Northern Tool store and pick up. Costs about $80.
