Rust Treatment for Steel: Remove, Convert, Prime, Protect

Rust on steel: check how deep it goes before you treat it

Rusted gates and grills, flaking paint on a shed truss, brown streaks from a railing, rusted rods showing in a slab: all are steel losing metal to water and air. Rust treatment for steel is a sequence of four stages, and the products for it are steps, not alternatives. A rust remover prepares the steel. A rust converter deals with rust that cannot be removed. A zinc rich primer protects the metal. An anti rust coating on top keeps the weather off the primer. Using one and skipping the rest is why rust comes back through new paint. Six checks tell you which stages your steel needs.

Find the right rust treatment for your steel

Match your steel to one route. Each route says what is done and takes you to its section.

Nothing loose stays under a coating. Every stage that follows depends on the one before it. The preparation decides how long the paint lasts.

Rust treatment finder for steel

Answer three questions for a starting recommendation, then confirm it with an Armstrong expert.

Rust remover or rust converter: how to remove rust from iron and steel

These two products are often confused. A rust remover dissolves rust and leaves clean metal that must then be primed. A rust converter is applied to rust that is left on the steel and turns it into a stable, moisture-repelling coating. They are not interchangeable. The remover comes first, wherever the steel can be cleaned to bare metal. The converter follows on tight rust that is still left.

Stage 1: remove everything loose

1. 1
   Degrease

   Remove oil, grease and dirt first. Chemicals and primers do not work through grease.Why: Primers and chemicals do not work through grease. They bond to clean steel.
2. 2
   Remove all loose, old and flaking material

   Wire brush, scrape, sand or power-tool the steel until all loose rust, mill scale and flaking paint are gone. Loose rust has no strength. Anything applied over it comes away with it.Why: Loose rust and scale lift the coating off with them when they fall away.
3. 3
   Wash off salt near the sea

   On marine and coastal steel, wash off salt contamination with high-pressure fresh water and let the steel dry. Salt left under a coating pulls moisture through it.Why: Salt left on steel pulls moisture through the coating, so rust returns beneath it.

Stage 2: remove the rust with ALSTRONG RR-170

1. 1
   Apply liberally

   Brush ALSTRONG RR-170 on generously so the rusted surface stays wet. Small parts, bolts and fasteners can be immersed.Why: The remover works only while wet, so the rust is kept wetted.
2. 2
   Leave it to react

   Allow 15 to 30 minutes.Why: The chemical needs time to dissolve the rust, and removing it early leaves rust behind.
3. 3
   Scrub and rinse

   Scrub with a wire brush, then rinse with clean water. This leaves a clean metal surface.Why: Rinsing leaves bare, clean metal and removes the reaction products.
4. 4
   Dry, then prime immediately

   Apply the primer as soon as the steel is dry. Clean steel left bare rusts again within hours in damp air.Why: Clean steel rusts again within hours in damp air, so it is primed straight away.

Stage 3: convert the rust that remains with ALSTRONG RC

1. 1
   Apply to the adhering rust

   After the loose rust is wire brushed off, apply ALSTRONG RC by brush, roller or sprayer over the rust that remains. It chemically stabilises the rust and converts it.Why: The converter changes the rust that is stuck on into a stable layer, so loose rust has to come off first.
2. 2
   Let the reaction finish

   The reaction is complete in about 30 minutes at room temperature. The data lists application between 15°C and 25°C.Why: The conversion takes time to complete. A topcoat applied early traps the unfinished reaction.
3. 3
   Apply a second coat

   A second coat is given for complete conversion.Why: A second coat finishes the conversion where the first was thin.
4. 4
   Follow with the next stage

   ALSTRONG RC is self-priming, so the converted surface takes the topcoat. On reinforcement, the bond coat and repair mortar follow.Why: The converted surface still needs a primer or topcoat to protect it from weather.

Zinc rich primer for steel: how it protects and how it is applied

Zinc is more reactive than iron. When both are in contact and moisture reaches them, the zinc corrodes first and the steel below is spared. This is the principle behind galvanising, and a zinc rich primer brings it to site as a coating. Even where the film is scratched, the zinc around the scratch goes on protecting the steel. Armstrong makes two zinc rich primers in the ALSTRONG range.

Both protect steel with zinc. They differ in make-up and in the exposure they are listed for, and either may be right for a job. Pick the one that fits the site, and an Armstrong expert will help you choose.

Applying ALSTRONG ZP 777

1. 1
   Prepare the steel

   Remove all loose rust, scale and flaking paint, then all dirt, dust, oil and contaminants. The primer penetrates and encapsulates tight rust, but nothing loose may stay.Why: The primer protects only where it touches clean steel, so rust, oil and scale are removed first.
2. 2
   Check the conditions

   Relative humidity must be below 85%. The steel must be at least 3°C above the dew point and not above 50°C. Drying slows in humid air, so avoid early morning dew and steel that is hot from the afternoon sun.Why: High humidity and steel near the dew point put moisture under the primer, which blisters it.
3. 3
   Stir

   If the product has settled, loosen it with a hand stirrer, then mix with a power stirrer until it is uniform.Why: Zinc settles in the tin, and a primer that is not uniform gives less protection.
4. 4
   Apply one coat

   Apply by brush or spray in a single coat at 25 to 35 microns dry film. The colour is light green with a matt finish.Why: The stated dry film thickness is what gives the stated protection. Too thin protects less, and too thick cracks.
5. 5
   Let it dry

   Surface dry in 1 hour and hard dry in 12 hours at 30°C. Full performance comes only after complete drying.Why: The primer reaches its performance only when completely dry.
6. 6
   Topcoat after 12 hours

   Wait at least 12 hours at 30°C. There is no upper limit, provided the surface is dry and clean when the topcoat goes on.Why: The topcoat bonds to a dry, clean primer. There is no upper limit if it is kept clean.
7. 7
   Work safely

   ALSTRONG ZP 777 is flammable, with a flash point of 30°C or above. Keep it away from flames and sparks and ventilate confined areas. Clean hoses, guns and spray equipment straight after use.Why: The primer is flammable, and solvent vapour in a closed space is a fire and health risk.

Applying ALSTRONG ZP PRIMER 210

1. 1
   Prepare to the stated standard

   Blast clean or wire brush the steel to Sa 2.5 or St 3 and remove all oil, grease and contamination. A zinc primer protects by contact with the steel, so the preparation matters more here than anywhere.Why: The standard sets the cleanliness and roughness the coating was tested on. A lower standard loses the stated life.
2. 2
   Mix and wait

   Mix Part A and Part B, then let the mix stand for 15 minutes before use.Why: The two parts begin to react on standing, and the wait gives the mix its full properties.
3. 3
   Apply within the pot life

   Apply by brush, roller or airless spray. Mix only what can be used within the pot life.Why: The mix stiffens as it cures, and a stiff mix cannot be applied evenly.
4. 4
   Apply the second coat

   Two coats are recommended.Why: Two coats give the full film thickness and cover the pinholes in the first.

New steel: prime it before it rusts

New fabricated steel is the simplest case, and it is where rust protection is most often skipped. Gates, grills, trusses, purlins, staircases and structural members are primed after fabrication and before they are fixed and exposed. Mill scale, weld spatter and flux are removed first, because they lift later and take the paint with them. After erection, the welds, bolts and any primer damaged in handling are cleaned and touched up before the topcoat.

Anti rust coating for steel: topcoats for gates, structures, sheets and tanks

A primer is not a finish. It is made to grip steel and protect it, not to face sun and rain for years. The topcoat keeps the weather off the primer, and the two together make the anti rust coating system. ALSTRONG ZP 777 can be overcoated with rubber, acrylic, PU, epoxy and water dispersion coatings. The topcoat is chosen for what the steel is and where it stands.

1. 1
   Wait for the primer

   Give ALSTRONG ZP 777 at least 12 hours at 30°C. The primed surface must be dry and clean.Why: A topcoat on tacky primer traps solvent and bonds poorly.
2. 2
   Apply the topcoat in full coats

   Apply the number of coats the topcoat calls for, covering edges, welds, bolt heads and the back faces. Rust starts where the film is thinnest.Why: Edges, welds and bolt heads rust first, so they get full coats.
3. 3
   Look at it every year

   Check before the monsoon. Clean and touch up chips and scratches before the rust spreads under the paint.Why: A chip or scratch lets rust spread under the coating, and touching it up early is far easier than repainting.

Iron pillars and columns: primer and a flexible top coat

Iron pillars rust from the surface in, and rust spreads under paint. A zinc-rich primer stops the corrosion at the metal, and a flexible top coat keeps rain off the primer. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Clean

   Clean the steel of dirt, dust and loose rust.Why: The primer must touch bare, clean metal to protect it.
2. 2
   Zinc-rich primer

   Apply one coat of ALSTRONG ZP 777, the zinc-rich epoxy primer.Why: The zinc protects the steel by itself corroding first, so scratches in the coat do not rust through.
3. 3
   Top coat

   Apply one coat of RAINSEAL RWC 902 diluted with about 20% water, so that the pillar is protected against rain as well as rust.Why: The flexible coat moves with the steel as it heats and cools without cracking.

Rebar rust treatment: exposed reinforcement and badly corroded steel

Steel inside concrete is normally protected by the concrete around it. When carbonation, chlorides, water and oxygen reach the bars, they rust. Rust takes up more room than the steel it came from, so it cracks the cover and pushes it off. That is the spalled ceiling or column with rusted rods showing. Patching the concrete without treating the steel fails, because the bar goes on rusting under the patch. This is the hard case on this page, and the steel treatment is one part of a structural repair.

Three ALSTRONG products are listed for coating reinforcement before a concrete repair. All three suit the job. The choice depends on the repair system and the site, and an Armstrong expert helps you choose.

1. 1
   Have the member assessed

   Spalling on a beam, column or slab is a structural matter. An engineer decides on propping and the extent of repair before concrete is cut away.Why: Spalling on a beam or column is a structural matter, so an engineer decides on propping before any work starts.
2. 2
   Break out the damaged concrete

   Remove all loose, hollow, cracked and carbonated concrete until sound concrete and bar that is not corroded are reached. Open the concrete behind the bar so the full circumference can be cleaned and coated.Why: Loose, hollow and carbonated concrete cannot hold a repair, and carbonated concrete no longer protects the steel.
3. 3
   Clean the steel

   Wire brush the bars until all loose rust and scale are gone, and clean the steel and concrete of dust, grease and soot.Why: Rust and scale under a primer lift it off, and the rust carries on under it.
4. 4
   Check the bars

   Where a bar has lost a marked part of its section, the engineer decides whether extra steel is added or the bar is replaced. This is the extreme step and is not needed on most repairs.Why: A bar that has lost section is weaker. The engineer decides whether to add steel.
5. 5
   Coat the steel

   Apply ALSTRONG ZP 777 in one full coat, or ALSTRONG AC in two coats within 3 to 4 hours of cleaning, or ALSTRONG RC. Coat the whole exposed length, including the back of the bar.Why: The primer protects the steel from rust and, in the concrete, from the chlorides and moisture that caused it.
6. 6
   Apply the bond coat

   Coat the prepared old concrete with ROOFBOND EP, or with a bonding slurry of ROOFBOND SBR 401 as in the Armstrong ceiling reinstatement procedure.Why: The bond coat joins old concrete to the new repair mortar, which would otherwise shrink away.
7. 7
   Place the repair mortar

   While the bond coat is still tacky, pack in ROOFCRETE EP or ROOFCRETE AR and compact it tightly around and behind the bars.Why: Mortar packed into a tacky bond coat bonds as one piece, and compaction removes the voids.
8. 8
   Cure

   Where a cementitious repair mortar is used, spray ROOFEXCURE WB curing compound straight after finishing.Why: Repair mortar needs to stay moist to reach its strength, and a curing compound keeps the water in.

The mortars, bonding agents and build-up depths are covered on the structural concrete repair page.

Structural steel that has lost section

A steel member with holes, deep pitting across its width or edges gone thin has lost strength. No rust converter or primer puts that back. An engineer measures what is left and decides whether the member is plated, strengthened or replaced. Coating comes after that, on sound steel, following the same stages as the rest of this page. Replacement is the last resort. Most rusted steel is caught long before that point and is saved by preparation, primer and topcoat.

Rust converter vs rust remover vs zinc rich primer: the ALSTRONG range

The ALSTRONG range from Armstrong covers each stage of rust treatment. The table sets the products side by side with their stated figures. Where more than one suits a stage, pick the one that fits the steel.

Rust treatment chemicals and products by stage

How much zinc rich primer for 100 sq.ft of steel

Approximate quantities worked out from each product's stated coverage, for 100 sq.ft of steel surface. Count every face that is coated: a plate has two, an angle or channel has more. The figures are for a plain surface, so rough and pitted steel and spray losses take more. For ALSTRONG ZP 777 the theoretical figure at 25 microns dry film is 16 sq.m per litre. Plan with the plain-surface figure of 1 litre per 60 sq.ft. An Armstrong expert confirms the final quantity after seeing the site.

For reinforcement, the surface is the bar length multiplied by its circumference. Ten metres of 12 mm bar has about 0.38 sq.m, or 4 sq.ft, of surface. The calculator works this out from the bar length and diameter.

Standards and test figures for rust protection of steel

• ALSTRONG RC states salt spray resistance of 340 hours at 25 microns to ASTM B 117, the standard salt fog test for coatings.
• ALSTRONG ZP PRIMER 210 calls for steel prepared to Sa 2.5 (blast cleaning) or St 3 (very thorough hand and power tool cleaning). These are preparation grades of ISO 8501-1.
• ALSTRONG ZP 777 states a dry film of 25 to 35 microns per coat, about 40% volume solids and a flash point of 30°C or above. Its data sheet lists no IS, ASTM or EN standard, so none is quoted here.
• CONPROOF FR 261 is applied to the dry film thickness the project fire rating requires. No fire rating figure is on file, so none is quoted here.

Rust treatment and primer quantity calculator

Enter the steel surface area, or the length and diameter of exposed bars, to see the coating needed. Quantities only.

Eight mistakes that bring rust back through new paint

1. Painting over loose rust. The paint is only as firm as what is under it.
2. Leaving cleaned steel bare overnight. Fresh rust forms before the primer goes on.
3. Using a rust remover and then no primer. A remover leaves clean metal with no protection.
4. Expecting a rust converter to work on thick, flaky rust. It converts adhering rust. Loose rust has to be brushed off first.
5. Skipping the topcoat. A primer left open to the weather wears out.
6. Priming damp or dew-covered steel. Humidity above 85% slows drying and weakens the film.
7. Missing the edges, welds, bolts and back faces. Rust starts where the coating is thinnest.
8. Patching spalled concrete without treating the bars. The steel keeps rusting and pushes the patch off.

Rust treatment for steel questions and answers