Coating Inspection: Film Thickness, Adhesion, and Defects

Coating inspection tells you whether a finish will do its job before it reaches a customer. Paint can look perfect and still be too thin to protect the steel, or look fine and peel off in a year. Inspection turns "looks good" into numbers, and those numbers let you fix a problem while it is cheap to fix. The tools are simple, but the results only count if you use them the same way each time and write down what you find.

What You Will Be Able to Do

• Measure wet film thickness and work out the wet film you need.
• Pick the right dry film gauge, verify it, and take a proper set of readings.
• Run tape adhesion and pull-off tests and report the results.
• Identify common defects, their likely causes, and their fixes.
• Check color and gloss and keep records that stand up to review.

What to Check and When

Most specs set hold points, where work stops until an inspector signs off. Do not coat over a step that has not been accepted.

Wet Film Thickness

Wet film thickness (WFT) is the thickness of the coating right after it goes on. It gives real-time feedback, so a painter can correct the film while it is still wet. A notch gauge, also called a comb gauge, is a flat piece of metal with teeth of stepped lengths (ASTM D4414).

1. Measure right after spraying, before the solvent flashes off.
2. Press the gauge straight down on a flat area until the end teeth touch the surface.
3. Lift it straight off and look at which teeth have paint on them.
4. Your reading is the highest tooth that came away wet.
5. Wipe the gauge clean right away, and repair the marks in the wet film if the spec requires it.

Working Out the Wet Film You Need

A wet film shrinks as solvent leaves. The volume solids on the data sheet tell you how much stays behind.

WFT = target DFT ÷ volume solids

Worked example: you want 3 mils dry, and the coating is 60 percent volume solids. WFT = 3 ÷ 0.60 = 5 mils wet. A mil is a thousandth of an inch.

Adding thinner lowers the solids, so you need more wet film. If you add 10 percent thinner by volume, WFT = 3 x 1.10 ÷ 0.60 = 5.5 mils.

Dry Film Thickness

Dry film thickness (DFT) is the thickness of the cured coating. It is the most important quality measurement in coating work. Too thin, and the coating will not protect the part. Too thick, and it may crack, peel, or fail to cure. Electronic gauges measure it without harm to the film.

Checking the Gauge

A gauge that has drifted gives you confident wrong numbers. ASTM D7091 covers the use and verification of these gauges.

1. Check that its calibration is current.
2. Verify it against known standards or shims at the start and end of each shift, in the range you will be measuring.
3. On a blasted surface, the rough profile can affect the reading, so follow your procedure for adjusting to the bare substrate.

Taking the Readings

For structural steel, SSPC-PA 2 is the usual method. It uses spot measurements: take about three gauge readings in a small circle and average them to get one spot. Take five spots in each 100 square feet. Individual gauge readings can vary, so the spots are what get compared with the spec. For production parts, your quality plan sets how many readings to take, commonly at least 3 to 5 per part, in set locations.

Worked example: a spec calls for 3.0 to 5.0 mils.

The area average is 16.9 ÷ 5 = 3.4 mils, so the area average is in range. Spot 3 is low. Whether it can stand depends on the limits your spec sets for a single spot. Mark the area, check around it to see how far the low film runs, and add coating if the spec requires it.

Adhesion Testing

Adhesion tests are destructive, so plan where to do them and how to repair them. Test on a witness panel or in an agreed location. The spec says which test, and what counts as a pass.

Tape Tests

Cross-hatch adhesion testing means cutting through the coating in a grid, pressing tape over it, pulling the tape off, and rating how much coating lifts. ASTM D3359 has two methods.

1. Pick a clean, dry area, and let the coating cure fully.
2. Cut through the coating to the substrate with a sharp blade or multi-blade cutter.
3. Brush away the loose flakes, then press the specified pressure-sensitive tape firmly over the cut area.
4. After about 90 seconds, remove the tape quickly, peeling it back on itself at as close to 180 degrees as you can. Do not pull straight up.
5. Rate the coating on the surface, not on the tape.

Ratings run from 5 (no coating removed) to 0 (severe removal), with A or B added to show the method, such as 4B. Ratings of 4 and 5 are generally good, but the spec sets the minimum. Record the rating along with the tape used.

Pull-Off Testing

A pull-off test (ASTM D4541) gives a number instead of a rating. You glue a metal dolly to the coating, let the adhesive cure, then pull it straight off with a portable tester. It reports the pull-off strength in psi or MPa. Always record where the break happened:

• Between the coating and substrate means an adhesion failure.
• Between two coats means an intercoat failure.
• Within one coat means a cohesive failure in that layer.
• In the glue means the coating is stronger than the number shows, and the test tells you only that it passed at that value.

Visual Defects

Check every part under good light. Name the defect, because the name points to the cause.

Holidays that the eye cannot see are found with a holiday detector, an electrical tool used on linings and other critical coatings. Use the voltage your spec calls for.

Runs and sags come from excess film thickness. Paint goes on too heavy and flows downhill before it sets. Dry spray and orange peel can look alike. Dry spray feels like sandpaper, because the droplets dried before landing. Orange peel is smooth to the touch but bumpy to the eye, because a wet film could not level out.

Color and Gloss

Color. Compare the part with the approved standard under the same light. Colors that match in the shop may not match outdoors. For a number, use a spectrophotometer, which reports the color difference as Delta E. If your spec allows a Delta E of 1.0 and you read 1.4, the part is out of spec, even if it looks close.

Gloss. A gloss meter shines light at a set angle and reports gloss units. Use the angle your spec calls for. Under ASTM D523, 60 degrees is the general angle, 20 degrees is used for high gloss, and 85 degrees for low gloss. Do not compare readings taken at different angles.

Before relying on either tool, check it against its reference tile, and measure on clean, flat areas.

Cure Checks

A film can look done and still not be cured. Depending on the spec, you might use a solvent rub test or a pencil hardness test. If a solvent rub softens or removes the film, or if a film marks easily, check the mix record and the cure time and temperature.

Inspection Records

If it was not written down, it cannot be proven. Records let you trace a failure years later, and they prove you met the spec.

• Job, part number, and area inspected.
• Date, shift, and inspector.
• Coating product, batch numbers, mix times, and thinner used.
• Air temperature, surface temperature, humidity, and dew point, with the times taken.
• Surface grade and profile readings.
• WFT and DFT readings, gauge serial number, and verification results.
• Adhesion test type and results, color and gloss readings.
• Defects found, repairs made, and who accepted the work.

Write the actual numbers, not "OK." Record them when you take them, not at the end of the shift.

Common Mistakes

Check Your Understanding

1. A coating is 75 percent volume solids, and the spec calls for 4.5 mils dry. What wet film do you aim for, with no thinner added?

WFT = 4.5 ÷ 0.75 = 6 mils wet.

2. A cured film measures about 3 mils on an aluminum panel. Which gauge do you use, and which adhesion method?

Use an eddy current gauge, because aluminum is nonferrous. For a film between 2 and 5 mils, use ASTM D3359 Method B with 6 cuts each way, spaced 2 mm apart.

3. A batch of parts shows small round craters on a line that was running fine yesterday. Maintenance just sprayed a lubricant nearby. What do you suspect?

Fisheyes from silicone or oil contamination. Stop, find what was sprayed, clean up the area, and test a part before running more.

4. A pull-off test reads 900 psi, and the dolly came off with only glue on it. What can you report?

The coating held at 900 psi at least. The glue failed, not the coating, so record the failure location and value.

Key Takeaways

• WFT is measured right after application, so you can correct the film while it is wet.
• DFT is measured on the cured coating, with magnetic gauges on steel and eddy current gauges on nonferrous metals.
• Verify your gauge, and follow the sampling plan in your spec.
• ASTM D3359 Method A is for films over 5 mils, and Method B is for thinner ones.
• The defect name points to the cause, so learn to tell them apart.
• Record real numbers at the time you take them.