Two Approaches to Rust
When rust (iron oxide) forms on steel surfaces, there are two fundamentally different approaches to treatment:
Rust Removal
Physically or chemically strips the rust layer away, exposing bare metal underneath. Methods include mechanical abrasion (grinding, blasting, wire brushing) and aggressive acid dipping.
Pros: Returns surface to bare metal. Suitable for re-fabrication or welding.
Cons: Bare metal is immediately vulnerable to re-oxidation (flash rusting). Mechanical methods damage surface profile. Acid dipping requires tank facilities.
Rust Conversion
Chemically transforms iron oxide into a stable, protective compound — typically iron phosphate (FePO₄). The converted layer adheres to the surface and inhibits further corrosion.
Pros: Passivates the surface. Provides a paint-ready primer layer. Can be applied in-situ without disassembly. Works on vertical/overhead surfaces with gel format.
Cons: Loose or flaking rust must be removed mechanically first. Very heavy rust may require multiple applications.
The Chemistry of Rust Conversion
Phosphoric acid (H₃PO₄) reacts with iron oxide (Fe₂O₃ — rust) to form iron phosphate (FePO₄):
Iron phosphate is a stable, insoluble compound that forms a thin, adherent grey-blue layer on the metal surface. This layer:
- Passivates the surface — prevents further oxidation by isolating the metal from moisture and oxygen
- Provides a primer base — excellent adhesion for paints, primers and protective coatings
- Inhibits under-film corrosion — reduces the risk of rust creeping under painted surfaces
- Is self-indicating — the visible grey/blue colour change confirms successful conversion
Why Gel Format?
Traditional liquid rust converters run off vertical and overhead surfaces before the acid has time to fully react with the rust. This results in:
- Incomplete conversion — patchy treatment with untreated areas
- Wasted product — most liquid drips off and is lost
- Multiple applications needed — increasing labour time and product consumption
- Runoff onto adjacent surfaces — risk of staining or damage to gelcoat, paint or deck finishes
CFS RustOff Gel uses a thixotropic (gel) formulation that clings to vertical and overhead surfaces. The gel maintains contact with the rust for the full 15–60 minute dwell time, ensuring complete and even chemical conversion in a single application.
When to Use Each Method
| Scenario | Recommended Method | Product |
|---|---|---|
| Deck hardware with surface rust | Gel conversion (in-situ) | CFS RustOff Gel |
| Structural steel before painting | Gel conversion | CFS RustOff Gel |
| Rust staining on gelcoat | Gel conversion | CFS RustOff Gel |
| Engine bay / bilge hardware | Gel conversion | CFS RustOff Gel |
| Fabricated steel before welding | Mechanical removal | Wire brush / grinder |
| Scale + rust inside heat exchangers | Acid circulation | CFS Descaler |
| Flash rust after acid descaling | Gel conversion | CFS RustOff Gel |
| Heavy flaking rust on hull plate | Mechanical removal + gel conversion | Wire brush + CFS RustOff Gel |
Step-by-Step: Rust Conversion with CFS RustOff Gel
Remove loose rust, flaking paint and heavy contamination with a wire brush or scraper. Degrease oily surfaces with an alkaline cleaner first.
Apply CFS RustOff Gel directly to the rusted surface with a brush, plastic spreader or cloth. Use a liberal coat — 1–3mm thickness for vertical surfaces.
15–20 minutes for light rust, 45–60 minutes for heavy oxidation. The gel will darken as it converts iron oxide to iron phosphate.
For heavy rust, work the gel into the surface with a stiff brush midway through the dwell time to improve conversion depth.
Rinse thoroughly with fresh water. The surface should show a grey/blue iron phosphate conversion layer. Repeat if rust remains. Prime or coat promptly after drying.
Frequently Asked Questions
What is the difference between rust removal and rust conversion?
Rust removal physically or chemically strips the iron oxide layer away, leaving bare metal. Rust conversion chemically transforms iron oxide into a stable compound (typically iron phosphate) that protects the surface and serves as a primer for paint. Conversion is generally preferred for marine applications as it provides ongoing protection.
Can I paint directly over phosphoric acid-treated surfaces?
Yes. The iron phosphate conversion layer formed by phosphoric acid gel is an excellent paint primer. After rinsing and drying, the surface is chemically stabilised and ready for coating. Apply primer or paint promptly after treatment to maintain the passivation benefit.
Why use a gel instead of a liquid rust converter?
Gel formulations like CFS RustOff Gel cling to vertical and overhead surfaces without dripping. This maximises acid contact time and ensures even treatment — liquid products run off before they can fully react, resulting in patchy conversion and wasted product.
Is CFS RustOff Gel safe on boat gelcoat?
Yes. CFS RustOff Gel is safe on fibreglass gelcoat, painted surfaces and most marine hardware at standard application rates. It is ideal for treating rust stains on gelcoat without damaging the surrounding surface.
How do I prepare the surface before applying rust converter gel?
Remove loose rust, flaking paint and heavy contamination mechanically using a wire brush or scraper. The gel is most effective when it has direct contact with the rusted metal surface. Degreasing with an alkaline cleaner before application improves results on oily or contaminated surfaces.
Get Started with CFS RustOff Gel
Available in 1L, 5L and 20L pack sizes. Contact us for pricing, technical advice or to request a sample.