What is Automotive Primer?

[ArcherLee]

Automotive primer is the first layer of paint directly applied to the surface-treated metal components. As the foundational layer of the entire coating system, it is designed to have excellent adhesion to the substrate and good compatibility with the intermediate or top coats applied above. Additionally, it must possess outstanding properties such as corrosion resistance, rust prevention, oil resistance, chemical resistance, and water resistance. Solvent-based automotive primers primarily use nitrocellulose resin, epoxy resin, alkyd resin, amino resin, or phenolic resin as the base material, with iron oxide red, titanium white, carbon black, and other pigments and fillers as coloring agents. The application methods include spraying and dip coating.

[StLawson]

The primer is the first layer of protective paint applied to the metal surface of the car body, just like putting on an undershirt for the car. I've seen too many rusty cars, all because the primer wasn't done properly. During factory application, the metal surface is thoroughly cleaned, and then epoxy or zinc-rich coatings are applied—these layers are particularly effective at preventing rust and corrosion. In daily driving, stones hitting the undercarriage or scratches can easily damage the primer. Once the metal is exposed, rust spots can spread within six months. When buying a used car, the first thing I check is the condition of the undercarriage—those with rust holes all stem from primer issues. New cars typically leave the factory with a primer thickness of about 20 microns. Don’t underestimate this thin layer—it adds several more years to the car’s lifespan.

[StHailey]

I've worked in the repair shop for nearly a decade, and the most common issue I deal with is chassis rust. The undercoating, which is the protective layer that isolates the vehicle's metal from the air, is what you feel when you touch the chassis—that gray or black coating. Most regular cars use epoxy undercoating, while off-road vehicles often have additional chassis armor for enhanced protection. Undercoating is most vulnerable to impacts and chemicals, like road salts, which accelerate deterioration. During repairs, we first sand off the rust spots with sandpaper and then reapply three layers: rust-proof primer, intermediate primer, and topcoat. Many car owners neglect the undercoating because it's out of sight, only to end up spending a fortune when the frame rusts through. I recommend checking the undercoating by lifting the car every quarter.

[ODakota]

At the last car enthusiasts' gathering, we talked about automotive primer and jokingly called it the car's 'sunscreen'. This coating is specifically designed to protect the metal frame, primarily composed of resin, anti-corrosion pigments, and solvents. When new cars roll off the production line, they first undergo an electrocoating bath to ensure the primer firmly adheres to the metal. Daily driving requires special attention to waterlogged road sections, as acid rain can gradually corrode edge seams. In northern winters when salt is used for snow melting, it's best to rinse the undercarriage after the season. For self-inspection, use a flashlight to check the inner wheel arches – if white powdery residue appears, it's a precursor to rust. When reapplying, choose stone-chip resistant primer and maintain an optimal thickness of around 0.3mm.

[McRhett]

As a seasoned driver, I believe the underbody coating is the true guardian of a vehicle. Composed of base materials, fillers, and additives, it forms a dense protective film. Having witnessed the entire electrophoretic painting process at the factory: the car body is immersed in the paint tank, and when electrified, the paint adheres to the metal like a magnet. This layer must withstand stone impacts, resist chemical corrosion, and endure temperature variations from sub-zero to 200 degrees. I recommend applying a glaze sealant to the underbody every two years, paying special attention to the seams around the exhaust pipe. When orange-peel-like rust spots appear on the frame, it indicates the undercoat has failed. Underbody armor can thicken the protective layer, especially suitable for vehicles frequently driven on rough roads.