This guide covers the full methodology — when each step applies, how to execute it, how to verify the result, and where the Two-Step Method is used in documented field applications.
Why Two Steps? The Problem With Heavily Corroded Steel
ISO 8501-1 defines four rust grades for steel surfaces:
| Rust Grade | Description | Surface Condition |
|---|---|---|
| Grade A | Mill scale covers the surface almost entirely. Little or no rust. | Bristle Blaster® can work directly — removes mill scale and creates anchor profile in one pass |
| Grade B | Rust has begun. Mill scale partially lifted and flaked. Some rust pitting visible. | Bristle Blaster® is effective; may benefit from initial cleaning pass |
| Grade C | Mill scale has rusted away entirely. Light rust pitting throughout the surface. | Two-Step recommended where laminar rust or scale build-up is present |
| Grade D | General pitting visible on the entire surface. Heavy, layered corrosion throughout. | Two-Step Method required — Tercoo® Step 1 is essential before Bristle Blaster® can achieve SP10 |
The distinction matters because the Bristle Blaster® is engineered to create a 65–85 µm Rz anchor profile on steel that is essentially clean but needs its surface activated. It is not designed to cut through laminar rust layers, remove thick scale deposits, or displace loosely adherent corrosion product. Attempting to use it directly on Grade D steel produces an inconsistent profile buried beneath corrosion residue — the anchor profile measurement is meaningless because the coating system will bond to the residue, not the steel.
Tercoo® solves this. It removes the corrosion mass so the Bristle Blaster® reaches clean steel at the right preparation stage.
Step 1: Tercoo® — Corrosion Removal
Tercoo® is a disc system that mounts on the same Bristle Blaster® drive unit used in Step 2 — one machine, two preparation steps, with a 30-second changeover between them. The Tercoo® disc uses flexible SBR discs with tungsten carbide pins that strike the surface in a controlled pendulum motion under centrifugal force — a hammering action that removes coatings up to 5 mm thick, heavy rust, tar, bitumen, epoxy and rubber layers from steel or concrete surfaces without generating heat. No grit containment is required.
What Tercoo® removes:
- Laminar and layered rust (the primary target on Grade C and D steel)
- Heavy mill scale deposits that have not responded to wire brushing
- Loosely adherent coating remnants over corroded steel
- Weld spatter and heat-affected oxide scale at weld seams
- Salt-contaminated rust crust over structural steel and pipeline surfaces
The result after Step 1: A clean, roughened substrate — cleared of bulk corrosion, thick coating and contamination, with the surface texture preserved for the Bristle Blaster® belt to create a consistent anchor profile. This is not yet a coating-ready surface; it is a surface ready for Step 2.
Step 1 is complete when: No loose or laminar rust is present. The surface is stable bare steel, even if stained or pitted. Running the Bristle Blaster® over the surface at this stage will produce a measurable, consistent anchor profile in the 65–85 µm Rz range on the cleared steel — rather than in residual rust.
Step 2: Bristle Blaster® — SP10 Finish
Once the Tercoo® pass is complete and the surface is cleared of heavy corrosion, the Bristle Blaster® is applied. The rotating wire tips impact the steel at high velocity, displacing remaining oxidation at the surface level and creating the angular anchor profile required for high-performance coating adhesion.
The Bristle Blaster® achieves on cleared steel:
- SSPC-SP10 / NACE No. 2 / ISO 8501-1 Sa 2½ cleanliness standard — minimum 95% of the surface free of all visible residues; SP5 / Sa 3 (white metal) achievable where required
- Anchor profile of 65–85 µm Rz — verified with ASTM D4417 Method C Testex replica tape; uniform roughness profile in accordance with ISO 8503
- Surface profile geometry comparable to abrasive blast cleaning — angular, not rounded
- ATEX Zone 1 and Zone 2 compliance (pneumatic model) — allowing use in explosive atmospheres where sandblasting is prohibited
The Bristle Blaster® cannot achieve these results when applied over uncleaned Grade D steel. After the Tercoo® Step 1 pass on the same drive unit, it consistently reaches SP10 — and SP5 / Sa 3 where the coating system demands it — in documented field applications across offshore, pipeline and heavy industry sectors.
Verification After the Two-Step Process
After completing both steps, verify the surface meets SP10 requirements before coating application:
Visual cleanliness — SSPC-VIS 1
Compare the prepared surface against SSPC-VIS 1 photographic reference standards for SP10. A minimum of 95% of the surface must be free of all visible oil, grease, dust, rust, coating, oxides, mill scale and foreign matter. The remaining 5% may show only slight staining — no rust, no scale, no residue.
Anchor profile — ASTM D4417 Method C
Press a Testex Press-O-Film® X-Coarse replica tape (range 40–115 µm) against the prepared surface and burnish firmly with the applicator — minimum 20 strokes in two crossed directions. Peel and place in a calibrated flat-anvil micrometer. Subtract 50 µm (tape substrate) to obtain the Rz value. Target range: 65–85 µm Rz. Take a minimum of three readings per representative area and record all results.
Soluble salt — ISO 8502-6 / ISO 8502-9
If the substrate history includes marine or industrial atmospheric exposure, measure soluble salt contamination with a Bresle patch (ISO 8502-6) and conductivity meter (ISO 8502-9). The coating manufacturer’s specification typically requires a maximum of 20–50 mg/m² chloride equivalent depending on the coating system and service environment. Salt contamination under new coatings is a primary cause of osmotic blistering — do not skip this step on previously exposed steel.
When Is the Two-Step Method Needed?
| Surface Condition | Recommended Approach |
|---|---|
| Grade A / B — mill scale with light surface oxidation | Bristle Blaster® directly. No Tercoo® step required. |
| Grade C — uniform rust, light pitting, no laminar deposits | Bristle Blaster® may be sufficient. Use Tercoo® if scale or layered corrosion is present in areas. |
| Grade C / D with laminar rust, thick scale or heavy deposits | Two-Step Method: Tercoo® first, then Bristle Blaster® to SP10. |
| Grade D — heavy pitting and layered corrosion throughout | Two-Step Method required. Bristle Blaster® alone will not achieve SP10 on this surface. |
| Old coating over corroded substrate (maintenance recoating) | Tercoo® to remove loose coating and rust. Bristle Blaster® to activate the steel beneath. Assess whether SP10 or SP3/SP6 is required for the new coating system. |
Documented Applications
| Application | Location / Operator | Condition Before | Result After Two-Step |
|---|---|---|---|
| Gas distribution pipeline — field joint preparation (1,100 km network) | GASNORP / Quavii, Piura, Peru (2021) | API 5L pipeline steel — field joint bare metal zones | SSPC-SP10 / Sa 2½ confirmed across pipeline corridor (independent inspection) |
| Pipeline field joints — high altitude, ATEX Zone 1 environment | Total E&P Bolivia, Planta Incahuasi (2021) | API 5L pipeline steel, high altitude — Tercoo® pre-treatment on corroded joints, Bristle Blaster® finish | SSPC-SP5 / Sa 3 achieved; soluble salts 1.4 µg/cm² — within Covalence heat-shrink sleeve acceptance threshold; ATEX compliance maintained |
Frequently Asked Questions
Can the Two-Step Method replace sandblasting entirely on a major maintenance project?
For structural steelwork, pipeline field joints and asset maintenance in the field, yes — the Two-Step Method achieves the same SP10 standard as abrasive blasting without the logistics, containment and environmental requirements that make blasting impractical or impossible in operational environments. For new construction in a fabrication shop with full blast facilities and containment, abrasive blasting remains the faster option at high volume. The Two-Step Method is the field and maintenance solution, not a fabrication shop replacement.
How long does the Two-Step process take compared to blasting?
On Grade D heavily corroded steel, the Two-Step Method is slower per square metre than blast cleaning at a properly equipped facility. In the field — on an operational pipeline, on offshore structural steelwork, at a remote maintenance location — the relevant comparison is not against a blast facility but against the alternative of mobilising blast equipment to a location where it cannot practically operate. In that context, the Two-Step Method is the only viable path to SP10.
Does Tercoo® need to achieve any specific cleanliness standard before the Bristle Blaster® is applied?
There is no formal intermediate cleanliness standard between the Tercoo® pass and the Bristle Blaster® pass — the Tercoo® step is preparation for the tool that achieves the measured standard, not a measured standard itself. The practical criterion is visual: no loose, laminar or flaking corrosion material remains on the surface. If the Bristle Blaster® operator encounters areas where the wire tips are rolling over loose material rather than impacting steel, the Tercoo® pass is not complete in that area.
Is special PPE required for the Two-Step Method?
Standard abrasion-resistant PPE applies to both steps: eye protection (safety glasses or goggles), ear protection (both tools produce significant noise), leather or cut-resistant gloves, and appropriate respiratory protection for the dust generated. In ATEX environments, ensure that all PPE is ATEX-rated. Neither Tercoo® nor the Bristle Blaster® generates the silica dust that is the primary respiratory hazard of abrasive blasting — however, rust and scale particles released during operation require appropriate respiratory protection.
Specify the Two-Step Method for your project
MontiPower technical team — tool selection, productivity estimates and application guidance for heavily corroded steel in your specific environment.