Most people tackle a rusty corrugated iron roof the same way: angle grinder, wire brush, primer, paint. It takes hours. It is dangerous work on a roof. And if a spot is missed, rust comes back through the paint within a season.
There is a faster approach — and a recent test on a galvanised iron roof demonstrated it works. The contractor applied the product, let it work, wiped it down, and painted directly over the surface. No primer. No grinding.
What the galvanised roof test showed
A contractor treated a rusted galvanised iron roof with Corrosion Assassin. No grinding. No sanding. No primer. The product was applied directly to the rusted surface, allowed to work, wiped down, and paint was applied straight over it. The result was clean adhesion and no rust bleed-through.
Why galvanised roofs need a different approach
Galvanised steel is coated in zinc. The zinc corrodes first to protect the steel underneath. When the zinc is depleted, the steel oxidises quickly.
Mechanical removal — grinding, wire brushing — strips what remains of the zinc coating. The problem is compounded while being fixed.
Hydrochloric acid-based rust removers carry the same risk. They are aggressive enough to attack the zinc coating itself, not just the rust.
How phosphoric acid works on rust
Corrosion Assassin is phosphoric acid-based. Phosphoric acid reacts selectively with iron oxide — it dissolves rust on contact. What it leaves behind is an iron phosphate layer: a stable surface that improves adhesion for the topcoat.
On galvanised steel, this means the rust is removed without stripping the remaining zinc. The surface is clean and ready for paint without a primer coat in standard applications.
Step-by-step process
- Clean the surface — remove loose dirt and debris with water.
- Apply Corrosion Assassin directly to rusted areas — brush, sprayer, or cloth.
- Allow dwell time — light surface rust: 10–20 minutes. Heavier rust: longer. Check and reapply if needed.
- Wipe down — remove dissolved rust residue with a cloth, or rinse with water.
- Inspect — surface should be clean and slightly matte. That is the phosphate layer.
- Paint directly — no primer required in most applications. Check topcoat manufacturer guidelines.
What you need
- Corrosion Assassin — 5L for a section, 25L for larger jobs.
- Brush, sprayer, or cloth.
- Water for rinsing.
- Your chosen topcoat paint.
- Standard roof safety equipment.
When this approach does not apply
If rust has eaten through the sheeting and there are holes or structural damage, no chemical product fixes that. Replacement sheeting is the only answer. Corrosion Assassin treats surface rust and mid-stage oxidation on structurally sound metal.
Keep reading: whether it is safe on galvanised and coated roofing steel, or see the Corrosion Assassin rust remover.
Want to test it on your parts?
We don't drop free samples. We run a 14-day controlled trial on a real batch, measured against your own baseline.