1. Why do the cut edges of galvanized color-coated coils rust easily, while the surface doesn't?
A: The surface of galvanized color-coated coils has a zinc plating layer (sacrificial anode protection for the steel base) and a color coating layer (isolates water and oxygen). However, the cut edge simultaneously damages both protective layers:
The zinc plating layer is cut off, exposing the bare steel substrate (iron) and depriving it of cathodic protection;
The color coating layer cannot cover the cut edge, allowing moisture and oxygen to directly contact the steel.
Therefore, the cut edge is a weak point in rust prevention and must be treated separately.
2. What is the most common method for rust prevention at cut surfaces?
A: Applying a high-zinc-content cold-galvanizing coating (such as an aerosol can or brush paint containing over 96% zinc).
Principle:
After drying, the coating forms an electrochemical protection (zinc preferentially corrodes, protecting the steel substrate);
It has a similar potential to the original zinc plating layer, avoiding accelerated corrosion due to potential differences.
Key points for operation: After cleaning and drying the cut surface, spray or brush evenly, covering the cut surface and surrounding area by 1-2 mm.
3. Besides cold galvanizing paint, what other alternatives or auxiliary methods are available?
A: Options vary depending on the working conditions:
Water-based inorganic zinc-rich primer: High temperature resistance, strong adhesion, suitable for cuts requiring subsequent welding;
Epoxy zinc-rich paint: Heavy-duty corrosion protection, suitable for marine or high-humidity environments;
Peelable rust-preventive oil: Temporary rust prevention (e.g., during transportation and storage), removable later;
Edge wrapping: Physically isolate moisture using waterproof tape, heat shrink tubing, or metal edging (in conjunction with sealant).
Note: Ordinary paint (zinc-free) only provides short-term protection; damage will accelerate localized corrosion.
4. What preparation is needed for the cut surface before painting? Can I just apply the paint directly?
A: Preparation is necessary; otherwise, the coating will easily peel off, leading to faster corrosion. Correct steps:
Remove burrs: Use a fine file or sandpaper to smooth the rough edges of the metal cut surface to prevent the coating from not adhering properly;
Degrease and remove contaminants: Wipe the cut surface and surrounding area with acetone or alcohol to remove cutting fluid and oil;
Slightly roughen: Use fine sandpaper (e.g., 400 grit) to roughen the surface of the cut surface to increase coating adhesion;
Dry: Ensure the surface is free of water and dust; use a heat gun to dry if necessary.
Never apply paint directly to an uncleaned or damp cut surface.
5. What should be done if the cut has already developed slight rust?
A: Treatment should be based on the severity of the rust:
Slight surface rust (not penetrating the steel substrate):
Remove rust with a wire brush or fine sandpaper → Clean → Apply a rust-converting paint (converts remaining rust into a stable complex) → After drying, cover with cold-dip galvanized paint.
Significant pitting (partial loss of steel substrate):
Grind until the metallic luster is exposed → Fill the pits with epoxy putty or metal filler → Grind smooth → Apply cold-dip galvanized paint or epoxy zinc-rich paint.
Severe perforation: Requires partial material replacement, or welding repair followed by re-treating the cut with rust prevention.