1.How to accurately determine the condition of the substrate?
Scenario A: Damage only affects the topcoat; the primer layer remains intact.
Appearance: A complete paint layer of a different color (usually primer) is visible underneath; the metal is not exposed.
Treatment: This is the simplest case. No metal primer is needed; simply sand and clean the surface, then apply a repair topcoat compatible with the existing topcoat.
Scenario B: The damaged area completely exposes the metal substrate. (Most common and requires more intensive treatment)
Appearance: A silvery-white (galvanized or aluminized zinc) or grayish-black (cold-rolled steel) metal surface is visible.
Treatment: A specialized "metal primer" must be used. This further depends on the substrate type:
Galvanized/Aluminized Zinc Plate: The surface has a dense zinc or aluminum-zinc alloy layer, making it difficult for ordinary primers to adhere and prone to peeling. A specialized "galvanized plate primer" (or "alloy primer") must be used. This type of primer contains special components such as phosphate esters that react with the zinc surface to form a strong bonding layer.
Cold-rolled steel sheet (uncoated): Conventional epoxy zinc-rich primers or epoxy ester primers can be used, which provide excellent rust adhesion and cathodic protection.
2.How to repair the most common situation B?
Recommended first choice (best compatibility, most reliable): Epoxy zinc yellow primer or epoxy ester primer for galvanized steel sheets
Advantages: Specifically designed for non-ferrous metals such as galvanized and aluminum alloys; excellent adhesion, effectively preventing coating peeling. Good corrosion resistance.
Applications: Local repairs of most galvanized/aluminized zinc color-coated coils.
3.Are there any other options?
Phosphating primer (wash primer): An extremely thin pretreatment primer that enhances the adhesion of subsequent coatings, but it has weak corrosion resistance and must be used in conjunction with other primers.
Two-component epoxy primer (general purpose): If it is uncertain whether it is a dedicated galvanizing primer, a high-quality general purpose epoxy primer also has strong adhesion, but its performance is not as stable as a dedicated one.
4.What types of paint are strictly prohibited from use?
Ordinary alkyd anti-rust paints (such as red lead paint and iron oxide red anti-rust paint). The fatty acids in these paints can undergo a "saponification reaction" with zinc, leading to blistering and large-area peeling of the coating.
5.What is the standard repair procedure?
Surface Preparation:
Smooth the edges of the damaged area with sandpaper (e.g., 180#-240#) to create a slope, removing rust and loose material.
Thoroughly clean the sanded area with a solvent (e.g., a special thinner or alcohol) to remove oil and zinc powder. This is crucial for adhesion.
Apply Primer:
Apply a thin, even coat of a special galvanized steel primer to the cleaned exposed metal area. The area can be slightly larger than the damaged area.
Strictly follow the product instructions for drying time and allow it to fully cure.
Apply Intermediate/Topcoat:
After the primer has dried, apply a repair topcoat that matches the color and gloss of the original topcoat as closely as possible. Polyurethane topcoats are generally recommended due to their good weather resistance and compatibility.
For better coverage and durability, two coats can be applied.
Overall Coating Protection (Optional but Recommended):
If the repair area is large or requires high precision, a thin coat of clear varnish (such as polyurethane varnish) can be applied to the entire repair area and surrounding area to unify the gloss and enhance protection.