Cleaning and etching metal such as unpainted galvanized steel and aluminum, are important preparatory steps to improve adhesion for subsequent paint coatings
A Phosphoric Acid solution works well as a metal etch. It is commonly used on bare metal such as galvanized steel and aluminum, to improve the adhesion and longevity of subsequent paint coatings. It also serves as a rust converter on ferrous metals by transforming ferrous iron into iron phosphate. Because this rust-inhibitive, iron phosphate coating is not very thick or resilient, proper paint coats should be applied before any new rust can form.
Always neutralize Phosphoric Acid by rinsing with clean water. Even though Phosphoric Acid has the ability to readily dissolve iron oxide, it etches iron very slowly allowing the user sufficient time to neutralize the surface without damaging the metal.
A Jasco product, “Prep & Prime” is an example of a Phosphoric Acid solution designed to be used for metal prep. It is sold in a diluted state with an acid concentration of 15.0-40.0%, according to their Material Safety Data Sheet (MSDS). This product has a dual purpose. As is the case with any Phosphoric Acid solution, it can be used for preparing metals such as galvanized metal and aluminum. It also serves as an effective rust converter for ferrous metals.
The proper use of this product as a metal etch requires additional dilution of 3 parts water to 1 part “Prep & Prime” according to their label instructions. Always add the acid solution to the water. Apply Prep & Prime with a paint brush or swab it with a rag, let stand for 15-30 minutes, and rinse with clean water. Always take adequate safety precautions when using any strong acid. Wear chemical resistant gloves for hand protection and chemical splash goggles for eye protection.
White vinegar is primarily composed of Acetic Acid and water. It typically contains less than 10% Acetic Acid by volume. Vinegar not only is an effective cleaner but it also has the ability to slightly etch galvanized metal and aluminum. One clear advantage of working with white vinegar is that it does not present the health and safety risks that stronger acids do.
Keep in mind that a primer coat is still required even though the metal surfaces have been etched.