How do you prepare steel to bare metal when you can't blast?
The vessel is alongside. The coating is lead-based. The repair is 6 m² in a live plant. Blasting is out, but the specification still says bare metal, and it still says a profile.
Use a power tool that cuts an anchor profile as it cleans, and specify SSPC-SP 11 rather than a blast grade. Rotary impact scarifiers cover 4-5 m² an hour on flat steel; rotary bristle tools reach welds and curves at about 1-3 m² an hour; needle scalers follow pitted geometry but leave no uniform profile. SP 11 asks for bare metal, no staining and at least 25 µm of profile. No single tool covers a whole structure.
Four ways to take steel back
Three power-tool families, plus the small-area blasting a site may still permit. Figures are the manufacturers' published numbers, not independent tests.
Rotary impact scarifier (rotopeen)
A hub at 2,000-2,700 rpm carries flaps with tungsten carbide shot bonded on. The shot fractures the coating and drives a profile into the steel, and never leaves the tool.
- Profile
- Up to 100 µm (Trelawny)
- Rate
- 4-5 m²/h
- Welds, edges
- Cutters only, never flaps
- Corners
- No, stops 55 mm off
- Loose media
- None, shot is captive
- Supply
- 30 cfm at 6.2 bar, or 230 V
Standard in lead paint removal and decommissioning. Extraction required, 2,000 W minimum. Trelawny PPT →
Rotary bristle tool
Steel bristles strike and rebound, removing corrosion and micro-indenting the steel in one motion. MontiPower's Bristle Blaster is the established product.
- Profile
- 65-85 µm Rz (MontiPower)
- Rate
- 1.1-3 m²/h
- Welds, edges
- Yes, its purpose
- Corners
- Yes
- Loose media
- None
- Supply
- Air, electric or cordless
The trade-off is throughput and consumable cost: a belt lasts about an hour.
Needle scaler
Reciprocating 3 mm needles follow whatever they are pushed against: pitting, corners, bolt heads, pack rust behind a bracket.
- Profile
- Not a profile method
- Rate
- Task-dependent
- Welds, edges
- Yes
- Corners
- Yes
- Loose media
- None
- Supply
- 4-15 cfm typical
Opens up the work. The pattern is hard to measure or make uniform, so do not certify profile with it. Needle scalers →
Small-area abrasive blasting
Vapour abrasive, sponge media, closed-cycle vacuum blasting.
- Profile
- Specification-dependent
- Rate
- 5-20 m²/h
- Welds, edges
- Yes
- Corners
- Yes
- Loose media
- Containment required
- Supply
- Compressor plus pot
On a large enough area it usually wins. It exists here because it is often not permitted, and mobilisation and containment can cost more than the preparation.
What SSPC-SP 11 asks for, and how it differs
Power-Tool Cleaning to Bare Metal is the standard most specifications use where blasting is not feasible. It asks for two things at once, and the profile is part of the standard, not left to the coating maker.
SSPC-SP 3
Power-tool cleaning. No bare metal required, and no profile. Plenty of maintenance systems in moderate exposure only need this.
SSPC-SP 11
A surface free of all visible oil, grease, dirt, mill scale, rust, paint and corrosion products, no staining anywhere, and a minimum 25 µm (1 mil) anchor profile.
SSPC-SP 15
Like SP 11 on profile, but permits staining across part of each unit area.
A surface can look bare and still fail on profile. Measure it: replica tape, depth micrometer or stylus instrument. Power tools leave repetitive, directional patterns, not the random pattern blasting leaves, so read across the tool's bias as well as along it.
Use the right pictorial standard. SP 11 is assessed against SSPC-VIS 3. The ISO 8501-1 photographs show blast-cleaned surfaces and give inaccurate results on power-tool work. Settle it with the inspector before the work starts.
And before any of it, check the coating's technical data sheet. Specifying SP 11 where SP 3 is enough buys preparation cost and nothing else.
No single tool finishes the area
The thing most product pages leave out, and the thing that turns a two-day job into a four-day one: pricing on the area tool's rate, then finding that 20% of the area is edges.
Flaps over bolts, rivets, weld seams or the edge of the work tear, damage the carbide tips, can create a dangerous condition, and the damage is not covered by warranty, per Trelawny's manual. Weld and slag work is done with star or beam cutters.
- Extraction is cooling, not just dust control. Trelawny gives poor vacuum flow as the cause of flaps burning round the carbide tips, and specifies a 2,000 W industrial vacuum minimum. Dust collectors.
- Cutters mark the steel. Star cutters remove thick coating fast and leave visible marks; beam cutters leave less. On thin-gauge or visible steel that is part of the specification.
- Two tools is normal practice, not a workaround: the scarifier for the field, a bristle tool, needle scaler or hand preparation for seams, edges and corners.
Will the coating manufacturer accept it? Usually: most warranties name a cleanliness grade and a profile range, not a method. But acceptance stays at their discretion, some systems specify blasting outright, and a power-tool profile is not identical to a blasted one. Ask their technical service in writing before the job, not after the first failed adhesion test.
Which tool for the job in front of you
- Flat areaDecks, tank walls, girders, floorsA rotopeen scarifier, if you have 30 cfm: fastest, and all its media stays captive. Trelawny PPT →
- Welds and curvesFillets, edges, pipeA rotary bristle tool, at roughly a third of the area rate. On seams and slag, cutters on a rotopeen drum are the other route.
- Pitted and irregular steelScale packed behind fittingsStart with a needle scaler to open it up, then finish to profile with one of the other two. Needle scalers →
Water blasting or mechanical scaling: which for your vessel?
UHP water jetting and mechanical scaling both strip coating and corrosion off marine steel. They behave very differently on cost, crew, water, flash rust and where they can physically run. Trelawny tested its mechanical route against 500-bar water blasting on hold and deck steel.
Coating-stripping rate in Trelawny's testing
- Consumable life
- 250-400 m²
- Flash-rust risk
- High: wet steel
- Manpower
- Trained crew, special PPE
In the same testing, around 130% more per hour
- Consumable life
- 750-1000 m² per drum
- Flash-rust risk
- Minimal: dry process
- Manpower
- One operator
Trelawny's own comparative testing. UHP jetting at higher pressures behaves differently again; rates and costs depend on the coating, pressure and equipment. For a planning rate on a deck, see deck descaling output.
The decision factors, compared
Cost per square metre is only the start. The practical differences show up in the water, the crew and the schedule.
| Consideration | Water blasting (HP / UHP) | Mechanical scaling |
|---|---|---|
| Cost per m² | Pump hire or ownership, fuel, jetting consumables, containment and water disposal, on top of labour. | An air tool plus wear parts, running off compressed air the vessel or yard already has. |
| Crew and kit | Pump unit, hoses and lances, trained operators, special PPE, usually several crew. | One operator per tool, minimal training, no pump plant. The kit travels in a case or on a pallet. |
| Water, runoff and MARPOL | Wash water with paint flakes and rust is a controlled waste stream to contain, collect and dispose of. | Dry debris captured at source by a dust collector. Nothing washes overboard. |
| Flash rust | Wet, freshly exposed steel flash-rusts before coating and often needs re-treatment. | Dry process, minimal risk: the surface can typically be coated without a re-treatment cycle. |
| Access and location | Best in the yard with pump support; awkward at sea, in accommodation, near cargo. | Anywhere the ship's air line reaches: afloat, holds, tanks, confined spaces, edges and fittings. |
| Finish for recoating | Strips whole systems back to bare steel and exposes the original blast profile. | A mechanically keyed surface; the PPT reaches SSPC-SP 11 with up to 100 µm profile. |
Full recoats and planned yard periods
- Large-area, back-to-bare-steel recoats in dry dock
- Yard infrastructure on hand: pumps, containment, disposal
- Coating specifications that explicitly require water jetting
Spot repair, maintenance afloat, water-restricted work
- Spot repairs and rolling maintenance at sea or alongside
- Holds, tanks, edges and detail a lance cannot work
- Near cargo, over electrical spaces, in ATEX zones: no wash water to contain
The pragmatic answer is both. Blast when the ship is in the yard for a full recoat and the specification demands it. Scale mechanically for everything in between. A modest kit on board, a deck scaler, a couple of needle scalers and a dust collector, keeps corrosion under control between dockings and shrinks the next blasting campaign.
Power-tool preparation: common questions
Can you achieve Sa 2½ with a power tool?
Not as a matter of standards. Sa 2½ is a grade defined in ISO 8501-1 for abrasive blast cleaning. Power-tool preparation is specified and assessed differently - SSPC-SP 11 against SSPC-VIS 3, or the ISO 8501-1 St grades for hand and power tool cleaning. Tool manufacturers often describe their output as "comparable to" Sa 2½, which is a reasonable description of the visual result and not a compliance claim. Specify the standard your inspector will actually measure against.
What surface profile do power tools produce?
Rotary bristle tools are published at 65–85 µm Rz. Rotopeen flaps are published by Trelawny at up to 100 µm. Both comfortably clear the 25 µm minimum in SSPC-SP 11 on sound steel. Actual profile depends on the substrate, the media condition and the operator, so measure rather than assume.
How fast is power tool preparation compared with blasting?
Roughly a quarter to a half the rate on flat area. Manufacturers' figures put blasting at 5–20 m²/hour, rotopeen scarifiers at 4–5 m²/hour and rotary bristle tools at 1–3 m²/hour. On small or awkward areas the comparison usually reverses once mobilisation, containment, media handling and clean-up are counted.
Do I need dust extraction for power tool surface preparation?
On lead-based paint or contaminated coatings, an extraction system with HEPA filtration is a legal requirement in most jurisdictions. On ordinary steel it is still strongly advisable, and on shrouded rotary scarifiers the manufacturer makes it a condition of correct operation - a minimum 2,000 W industrial vacuum, or the consumables overheat.
What size compressor do I need for a rotopeen deck scaler?
Around 30 cfm at 6.2 bar for a Trelawny PPT, in either cutting width, with a hose bore of at least 13 mm and a run no longer than 30 m. Each additional 15 m of hose costs roughly 0.21 bar. Note that Trelawny's web specification table lists this figure as "30 l/s (840 cfm)" - their datasheet and manual both give 30 cfm, which is the number to size against.
Does power tool cleaning affect the coating warranty?
Most coating warranties specify a cleanliness grade and a profile range rather than a preparation method, so a compliant power-tool-prepared surface is normally acceptable. Acceptance remains at the manufacturer's discretion. Confirm with their technical service before application.
Is water blasting faster than mechanical scaling?
Not on decks, in Trelawny's own testing. Stripping coatings with a TFP200 mechanical deck scaler ran at up to 30 m²/hr against around 13 m²/hr for 500-bar water blasting, and the mechanically scaled steel comes up dry, with no wash-down or drying time before the next step. Ultra-high-pressure jetting has its place on full recoats in the yard, but for hold and deck descaling the dry mechanical route was around 130% more productive per hour, as reported by Trelawny.
Which method is right for spot repairs and maintenance at sea?
Mechanical scaling. A needle scaler or handheld deck scaler runs off the ship's own compressed air, needs one operator, brings no high-pressure pump or water handling on board, and works exactly where the corrosion is: edges, fittings and patches included. That is why crews carry Trelawny tools for maintenance afloat and keep blasting for planned, large-area work in the yard.
Keep planning
Tell us what you have to prepare
We spec the tool and the consumables against it, and say which parts of the area it will not reach. SepcoTech is the authorised Trelawny distributor, quoting in EUR with no hidden import costs.
- Substrate
- Coating system
- Geometry
- The standard in the spec
- Area
- Timeframe
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