More Profile Isn't More Grip

The Wet Edge, No. 003. Short field notes for coating crews.

There's a feeling on a lot of blast jobs that deeper is better. If 2 mils is good, 4 must be great. It isn't.

Profile gives the primer something to grab, and the range on the spec or data sheet is the target, not the deepest you can get. Go past the top of it and the problem isn't the average reading. It's the tallest peaks. ASTM D4417 even notes that coating performance in a small area is driven by the highest features, not the average. Those rogue peaks can poke through a thin primer, and that's where pinpoint rust starts.

It costs you twice. First in material, because the deeper valleys soak up coating that never counts toward your DFT. The gauge reads from the peaks. Then in callbacks, when rust dots show up across a wall that passed inspection.

The practical move: profile comes mostly from abrasive size and nozzle pressure. Pull a replica tape reading at the start of the shift with the abrasive and pressure you're actually running today. If you're running high, step down a grit size before you've blasted the whole wall. If you're already over, ask the coating manufacturer what they'll accept before you just add mils.

Bottom line

Stay inside the profile range on the spec or data sheet. The tallest peaks decide where rust starts, and extra profile burns material that never shows on the gauge.

Further reading: ASTM D4417, Field Measurement of Surface Profile of Blast Cleaned Steel. Standards: ASTM D4417, SSPC-PA 17.

Talk to a consultant. Questions on a job? Atlantic Coating Consultants supports contractors in PA, NJ, DE, MD, DC, WV and OH with product recommendations, field support and crew training. Call 484-644-5303 or email atlantic@tnemec.com.