What is the best material for patching rust holes in cars?
4 Answers
You can purchase galvanized steel plates with excellent anti-corrosion properties to weld and repair the damaged areas. Here is some knowledge about car rust: 1. Introduction: Rust is a chemical reaction that occurs when iron is exposed for a long time. Iron is prone to rusting due to its active chemical properties and significant influence from external conditions. Moisture is one of the primary factors that cause iron to rust. 2. Principle: Water alone does not cause iron to rust. Only when oxygen from the air dissolves in the water, reacting with iron in a moist environment, does it form iron oxide, commonly known as rust. Rust is a reddish-brown substance that is not as hard as iron and easily flakes off. When a piece of iron completely rusts, its volume can expand up to 8 times. If rust is not removed, this spongy rust readily absorbs moisture, causing the iron to deteriorate even faster.
Last time, there was a coin-sized rust hole on my car's rocker panel, and it took visits to three repair shops to figure it out. The experienced mechanic gave the most reliable advice: For strength and durability, cut out the rusted metal and weld in a new steel plate, then apply epoxy rust-proof primer, fill with putty for leveling, and finally spray paint—this can last seven or eight years without recurrence. For small holes or temporary fixes, use an automotive-specific fiberglass repair kit, which includes resin and fiberglass cloth. Mix and apply it like plastering a wall, then sand and paint after it cures. Never cut corners by just using regular body filler or AB glue—rainwater will still cause rust, and I personally tested it; it bubbled and peeled within six months. The key is thorough rust removal; grind with a wire wheel until it shines.
I've repaired dozens of rusty cars, and to be honest: For small rust holes, aluminum alloy repair agents are the most convenient option. You can get a set for 30-40 RMB on Taobao, and it can still be drilled and tapped after curing. Large holes must be handled in layers—apply rust conversion paint at the bottom to block oxygen, fill the middle with fiber-reinforced body filler (ordinary putty will shrink and crack), and then seal the surface. I've seen people use soda can lids as patches, which works in the short term, but different metals in contact will cause electrolytic corrosion. A more professional approach is to use galvanized cold-rolled steel sheets cut into patches, which are more rust-resistant than the car's original sheet metal. When welding patches, remember to spot weld to avoid burn-through, and apply weld seam anti-rust agent immediately after welding.
A buddy from the chemical lab taught me a neat trick: first spray rust converter to turn red rust into black phosphating film, then apply epoxy metal filler mixed with stainless steel powder. This material conducts electricity and prevents electrochemical corrosion, making it especially effective for repairing exhaust pipes. If you can't use heat near plastic parts, use polyurethane structural adhesive as a base, cover it with galvanized mesh as a framework, and apply automotive-grade fiberglass filler on the surface. However, the most reliable method is still cutting and welding. Repair shops use MIG welding with copper backing pads, which can even penetrate 0.3mm thin sheet metal without burning through, and the repaired part sounds just like the original when tapped.
