Abrasive Blasting Practice and Surface Profile Measurement

Why Surface Preparation Is the Foundation of Every Coating System

No matter how advanced a marine coating is, it will fail prematurely if the steel beneath it is not properly prepared. Coating manufacturers routinely state that surface preparation accounts for the majority of coating failures in the field. In shipbuilding and marine fabrication, where hull steel is constantly exposed to seawater, humidity, and electrochemical corrosion, getting surface prep right the first time is not optional — it is the job.

This lab focuses on abrasive blasting, the method used on virtually every production vessel, and on measuring the anchor profile the blasting creates. Both skills are testable on NCCER Shipfitting, NCCER Maritime Welding, and ABS/USCG qualification records.

SSPC-SP 10 Near-White Metal Blast Cleaning

SSPC (Society for Protective Coatings) publishes the industry-standard surface preparation specifications used throughout US shipbuilding. The specification you will work to today is SSPC-SP 10, Near-White Metal Blast Cleaning.

SP 10 requires that, when viewed without magnification, the surface must be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, and foreign matter — with the exception that staining (shadows, streaks, or discoloration) is permitted on no more than five percent of each square inch of surface area. This makes SP 10 one step below SSPC-SP 5 (White Metal), but far cleaner than SSPC-SP 6 (Commercial Blast). Most marine topcoat and epoxy primer systems call for SP 10 or better.

To judge whether you have reached SP 10, you compare your blasted panel side-by-side with the SSPC VIS 1 photographic reference standard. VIS 1 contains color photographs of steel at various blast levels starting from different initial rust grades (A through D). Match your panel to the correct SP 10 photo for your starting rust condition. If residual staining covers more than five percent of the surface, continue blasting.

Blasting Equipment and Technique

A blast pot stores abrasive media under pressure and meters it into the air stream through a metering valve. The air-and-media mixture travels through a blast hose to a nozzle held by the operator. Today you will use garnet, a hard, angular mineral abrasive widely used in shipyards because it produces a good anchor profile, breaks down cleanly, and generates less free silica than some other media. Never use silica sand for abrasive blasting; it creates respirable crystalline silica dust, which causes silicosis, an irreversible and potentially fatal lung disease.

Correct blasting technique requires attention to three variables:

• Nozzle angle: Hold the nozzle at approximately 45–90 degrees to the work surface. A perpendicular (90°) angle removes rust aggressively; a shallower angle can be used for feathering edges.
• Standoff distance: Typically 12–18 inches from nozzle tip to surface. Too close concentrates energy on a small spot; too far reduces cutting power and leaves an uneven profile.
• Overlap pattern: Work in consistent parallel passes with each pass overlapping the previous one by roughly one-third to one-half the blast width. Irregular patterns leave skips and high-stain areas.

Deadman switch: All blast nozzles used in production settings are fitted with a deadman (dead-man's handle) safety device. The operator must maintain continuous grip pressure to keep the abrasive flowing. Releasing the handle immediately stops the blast stream. Confirm the deadman functions before each use — this is a pre-blast checklist item.

Pre-Blast Safety Checklist

Blasting must not begin until every item below is verified:

1. Supplied-air helmet fitted, airline connected, airflow confirmed at the helmet

2. Tyvek coverall, leather gloves, and steel-toed boots worn

3. Containment sheeting secured around the blast area to prevent media and debris migration

4. Ventilation confirmed — exhaust must carry dust away from the operator and bystanders

5. Air quality at the supplied-air source verified (breathing-air-grade supply, not shop air without a carbon monoxide monitor and filtration)

6. Deadman function tested by releasing grip and confirming blast stops

7. Instructor authorization received before the blast pot is pressurized

Any unsafe act — removing the helmet while blasting, pointing the nozzle at another person, bypassing the deadman — results in an immediate work stoppage and mandatory safety debrief before any student may resume blasting.

Anchor Profile and Why It Matters

Abrasive blasting does not just clean the surface — it roughens it. The microscopic peaks and valleys left behind are called the anchor profile (also called surface profile or blast profile). Coating manufacturers specify a minimum and maximum profile depth because:

• Too little profile: the coating has insufficient mechanical tooth to adhere reliably.
• Too much profile: the peaks may protrude above a thin coating film, creating rust points.

Profile depth is measured in mils (thousandths of an inch). A typical heavy-duty marine epoxy primer might call for 1.5–3.0 mil, which is the target range in today's lab.

Measuring Profile with Testex Press-O-Film Replica Tape

The Testex Press-O-Film system is the most widely used field method for measuring anchor profile. It consists of a compressible foam backed with a thin incompressible polyester film. When you press the tape firmly onto the blasted surface, the foam conforms to the peaks and valleys. You then measure the total thickness of the tape with a spring micrometer (also called a dial micrometer or Testex gauge) and subtract the known thickness of the polyester backing (50 micrometers, or 2 mil for the standard product — but always verify with the tape manufacturer's instructions, as tape grades vary). The remainder equals the profile depth.

Tape grades: Use Coarse tape for profiles roughly 0.8–2.5 mil and X-Coarse (Extra Coarse) for profiles roughly 1.5–4.5 mil. When in doubt about which grade to use, start with Coarse; if the reading appears near or above its upper limit, switch to X-Coarse.

Standard practice is to take a minimum of two readings at different spots on the panel, average them, and record both individual readings plus the average on the inspection form. The average is the value compared to the specification.