Abrasive Blasting

Abrasive blasting propels hard particles at high velocity against a surface to remove rust, mill scale, old coatings and other contaminants. It is often still called sandblasting, even though silica sand is now rarely used. On steel it does two jobs at once: it leaves a clean surface, and it cuts a surface profile, the microscopic peaks and valleys that give a coating mechanical “tooth”.

The substrate is the foundation of every coating system, and many premature failures trace back to poor preparation rather than to the coating itself.

How abrasive blasting works

Compressed air, or in shop equipment a spinning wheel, accelerates abrasive particles toward the work. Angular particles strip away mill scale, rust and coatings, and cut the steel itself to create the profile. The result is judged on two separate criteria:

• Cleanliness is how much rust, mill scale, coating and staining remains. It is specified with grades such as SSPC-SP 10/NACE No. 2 (near-white metal) or ISO 8501-1 Sa 2½. See SSPC, NACE and ISO surface preparation standards.
• Profile is the peak-to-valley depth of the anchor pattern. It is specified as a range in mils or micrometres and measured by one of the methods in ASTM D4417.

A third criterion is freedom from contaminants you cannot see, such as chlorides. Visual cleanliness grades do not cover it, so it has to be specified and tested separately (see soluble salt contamination).

Blasting methods

Dry open-nozzle blasting

The most common field method: a blast pot meters abrasive into a high-velocity air stream directed through a hand-held nozzle. It is fast and versatile but dusty, and needs containment where debris or old coatings are a concern.

Wet abrasive and vapor blasting

Water is added at the nozzle or as a premixed slurry. This greatly reduces airborne dust and helps remove soluble salts, but the steel may flash-rust. Any corrosion inhibitor added to the water must be approved by the coating manufacturer, and the slurry has to be collected.

Centrifugal wheel blasting

Shop machines use turbine wheels to throw recycled steel shot or grit at plate, structural shapes and pipe. It is efficient and highly repeatable, but only for parts that fit through the machine.

Vacuum (closed-loop) blasting

A shrouded blast head recovers abrasive and debris at the surface. Production is low, so it suits spot repairs and sensitive environments.

High- and ultrahigh-pressure water jetting (SSPC-SP WJ/NACE WJ series) removes coatings and soluble salts well, but it only re-exposes existing profile; it cannot create new profile.

Choosing an abrasive

Abrasives differ in hardness, shape, density, breakdown rate and recyclability. Angular particles cut a sharp profile; round shot peens the surface into a smoother, dimpled texture.

Particle size is the biggest single control on profile depth. Coarse abrasive cuts deeper but delivers fewer impacts per pound, so it cleans more slowly; finer abrasive cleans faster with a shallower profile. Recycling systems must maintain a balanced “work mix” of sizes.

Abrasive must also be clean. Requirements are set by SSPC-AB 1 (mineral and slag), SSPC-AB 2 (recycled ferrous metallic) and SSPC-AB 3 (ferrous metallic); ASTM D4940 checks soluble contamination by conductivity and ASTM D7393 checks for oil.

Equipment, nozzle pressure and air quality

A typical open-nozzle setup has a compressor, aftercooler and moisture separator, blast pot with metering valve, blast hose and a venturi nozzle. Productivity depends mainly on pressure at the nozzle, commonly about 90–110 psi (6.2–7.6 bar) for steel. Long or undersized hose, extra couplings and worn nozzles all cut that pressure, so the compressor gauge can be misleading.

Figures are approximate for new nozzles. A worn nozzle uses more air and is commonly replaced once the bore is about ¹⁄₁₆ in (1.6 mm) oversize. Size the compressor from the manufacturer’s data with a margin for wear and line losses.

Compressed air must be free of oil and water. The ASTM D4285 blotter test discharges air onto clean white absorbent material and checks for staining.

Step-by-step blasting procedure

1. Pre-clean. Remove oil, grease and similar contaminants by solvent or detergent cleaning per SSPC-SP 1. Blasting spreads them instead of removing them.
2. Correct fabrication defects. Round sharp edges and remove weld spatter, slivers and laminations where the specification calls for it. ISO 8501-3 defines preparation grades P1–P3 for these imperfections.
3. Check conditions. Confirm that the steel temperature is at least 5 °F (3 °C) above the dew point and that humidity is within the specified limit. See environmental conditions for coating.
4. Verify air and abrasive. Run a blotter test and confirm that the abrasive meets the specified cleanliness and size.
5. Blast. Hold the nozzle at a steady distance, commonly around 18–24 in (450–600 mm), roughly perpendicular to the surface. Overlap passes until the specified grade is uniform.
6. Remove dust and spent abrasive. Blow down with clean, dry air or vacuum, working from top to bottom. Assess residual dust with the tape test in ISO 8502-3.
7. Inspect. Compare the surface against the written standard, using visual references such as SSPC-VIS 1 as an aid. Measure the profile and run soluble salt tests if the specification requires them.
8. Prime promptly. Apply the primer before any visible rust bloom or contamination forms. If the surface has degraded, re-blast it to the specified grade.

Safety and environmental controls

Blasters need NIOSH-approved abrasive-blasting supplied-air respirators (Type CE in the US) fed with breathing-quality air, plus hearing protection and protective clothing. US rules require a deadman control that stops the blast when released. Routine controls also include whip checks and pins on hose couplings, static grounding and keeping others clear of the blast path.

Test old coatings before removal. Lead, chromates and other heavy metals trigger exposure rules, containment (SSPC Guide 6 gives a framework) and waste characterization; contaminated spent abrasive may be hazardous waste.

Frequently asked questions

Is abrasive blasting the same as sandblasting?

“Sandblasting” is the everyday name for the same process. However, true silica sand is now rarely used because of its silicosis risk. Modern work uses slags, garnet, steel grit, aluminum oxide, crushed glass and other media.

How soon must blasted steel be coated?

There is no single number. Most specifications require priming before visible rust or contamination appears, which in practice usually means the same shift. High humidity shortens that window. If rust bloom appears, re-blast before coating.

Can you blast when humidity is high?

You can blast, but the steel will flash-rust quickly if its temperature approaches the dew point. Many contractors control the environment with dehumidification so that blasting and priming can continue in damp weather.

What is the difference between abrasive blasting and water jetting?

Abrasive blasting cleans the surface and creates new profile. Water jetting removes coatings, rust and soluble salts but leaves only the profile that already existed. It is often chosen for maintenance work on previously blasted steel.