1. Surface with Residual Contamination (Unclean Surfaces)
Oil/grease residues: Even small amounts of oil (from tools, gloves, or coolant) create a barrier between the coating and steel. The new coating will not bond, leading to immediate peeling.
Unremoved loose rust/flaky old coating: If peeling old coating, powdery rust, or scale is left on the surface, the new coating will adhere to these unstable layers instead of the base steel-resulting in cracking or detachment over time.
Chemical residues: Residual acids (from previous pickling), alkalis (from improper neutralization), or salts (coastal salt spray, road salt) will react with the new coating, causing discoloration or micro-corrosion under the film.
2. Surface with Excessive Damage or Deterioration
Severe pitting corrosion: Deep, widespread pits (depth >1mm) trap moisture and contaminants that cannot be fully removed by grinding. The new coating will not seal these pits, leading to hidden corrosion that destroys the film from within.
Thinned steel structure: If repeated grinding (from prior reapplications) has reduced the steel's thickness below design limits, reapplication is pointless-structural integrity is compromised, and corrosion will continue to eat away at the base material.
Cracked or deformed surfaces: Structural cracks (even small ones) or mechanical deformation create stress points where the new coating will crack first, failing to provide uniform protection.
3. Surface with Incompatible Pre-Existing Layers
Non-breathable old coatings: If the original coating is a non-breathable type (e.g., epoxy, fully sealed polyurethane) that hasn't been fully stripped, the new rust conversion coating (which requires breathability) will trap moisture between layers-causing blistering and delamination.
Mismatched chemical layers: Residual chromate-based coatings, galvanizing zinc layers, or paint with strong solvents will react with the rust conversion agent (e.g., tannic acid, phosphoric acid), leading to chemical incompatibility and coating failure.
4. Moist or Improperly Prepared Surfaces
Wet or damp surfaces: Any residual moisture (from cleaning, rain, or high humidity) on the surface will mix with the conversion coating, creating micro-bubbles and reducing adhesion.
Over-grinded or overly smooth surfaces: Excessive grinding (Ra <0.5 μm) creates a too-smooth surface that lacks mechanical interlocking for the coating-leading to poor bonding and easy peeling.
Unneutralized rust: Heavy, adherent rust that hasn't been neutralized or converted will continue to corrode under the new coating, breaking down the protective film.
5. Surface in Harsh Environmental Exposure (Before Curing)
If the surface is exposed to continuous rain, salt spray, or extreme temperatures (<10°C or >35°C) during or immediately after reapplication-even if the surface is clean-the coating cannot cure properly, resulting in incomplete film formation and reduced durability.
