By Jack Bronson
As iron is not found in metallic form because of its reactivity with water, oxygen, and other elements or componds, it is safe to assume that what we call rust did not show itself to man until after the start of the iron age, when metallic iron was first produced by smelting. And it has been a major problem since then.
The removal of rust is a simple chemical process that can be effected with a variety of substances. What creates difficulties is the size and shape of the object to be cleaned. The degree of rusting influences the approach. Where possible grinding and media blasting can remove much of the worst rust. However the use of acid dipping can be effective with complex shapes.
There are many acids that will do the job. For heavily rusted items, Hydrochloric acid (Muriatic acid), will agressively dissolve rust and unrusted metal at the same time, and will go on reacting until all the acid is used up. The object should be thoroughly washed to remove residues, and in some cases to be extra cautious, a further rinse with dilute alkali (caustic soda or ammonia) and then another water rinse will leave a clean surface for the next step, which is the same as for lightly rusted items. For this Phosphoric acid is strongly recommended. Use about 25% concentration. You can buy this in various ways. Some hardware stores sell small bottles. Autoparts stores sell magnesium wheel cleaners, which are mostly phosphoric acid. Laboratory supply houses sell technical grade in larger quantities. This is often 85% and can be diluted one part acid and two parts water. Be careful! Add acid to water not water to acid. Wear protective clothing and safety glasses.
Why Phosphoric acid? There are two reasons. The reaction of first dissolving the rust and then the surface of the metal will leave a coating of iron phosphate crystals on the surface of the metal. These crystals are attached to the metal by atomic bonds which are much stronger than the bond of adhesion of paint to metal. The second reason is that the crystals create a much larger surface area for primer to adhere to. To explain this, consider a cube of 1" side. The total surface area of the cube is 6 sq.in. Now divide the cube along each side into 1/10" and you have 1000 cubes 1/10" x1/10" x 1/10". Each cube has a total surface area 6/100 sq.in. But as there are 1000 cubes the total surface area is 60 sq.in. Obviously this analogy only gives an idea of how the surface area for paint adhesion can be increased with phosphate crystals. The next step is critical. Do NOT wash the treated surface with water. This will dissolve the crystals. There can be a gummy residue. Wash it with methyl alcohol (methyl hydrate) using a soft toothbrush to reach into crevices.
The resulting surface is stable except to water. In a dry atmospere it will last for months with no rusting, but it makes sense to prime as soon as possible. I like metal etching primers, because I believe the name implies some phosphoric acid content, but I have never been able to find out if this is true. Your preferred primer and paint system will work because the most critical preparation has been done.
It is interesting to note that the most ignorance and misunderstanding of basic rust prevention is displayed by body shops, especially in collision repair. How often does an accident display the whole paint system lifted off the bright shiny metal in one sandwich, showing the primer had never properly attached to the metal?