Summary: Where blasting is impossible, 'coating over rust' relies on hand/power tool cleaning (St2/St3) to remove loose rust and mill scale, plus rust converters/stabilisers on thin residual rust, reducing re-rusting.
Mechanism
Loose rust must be removed; only thin, well-adhered residual rust may be converted. Converters (tannic/phosphoric) turn iron oxide into a stable chelate as a pre-treatment, not a substitute for cleaning.
Key data
- St2 thorough / St3 very thorough cleaning decide adhesion and re-rusting risk.
- Apply the system promptly after conversion to avoid flash rust.
- Rust-tolerant epoxy tie coats help adhesion on low surface profile but do not replace removing loose rust.
Cleanliness
| Grade | Method | Fit |
|---|---|---|
| St2 | Hand / power tool | Maintenance, thin coats |
| Sa2.5 | Blast (not over-rust) | Preferred for heavy duty |
Defects & fixes
- Re-rusting after coat: loose rust not removed — rework to St3.
- Chalky/loose conversion layer: uneven or over-acid — thin even coat, flash.
- Poor adhesion: oil or soluble salts — degrease and desalinate first.
FAQ
Is coating over rust the same as no prep?
No — remove loose rust, scale, oil and salts; only thin adhered rust can be converted.
Do converters give long life?
They are a pre-treatment needing a full system on top; alone they are not durable.
Can't blast on site?
Use St3 power tooling + tie coat + converter; still lower than blasting, expect shorter life.
Ref: GB/T 8923 / ISO 8501 cleanliness grades; rust-conversion practice (public).
Kexin New Materials (Guangdong) Co., Ltd. · 2026-09-18