Thin, Dry, Pinholey - The Airless Rig That Couldn't
You are inspecting a 100% solids epoxy lining for a potable water tank: 20–25 mils DFT, single coat, applied by plural-component airless spray, mixing ratio 2:1 by volume. The contractor's rig runs a 68:1 pump with 3/8 in ID by 150 ft of hose, no heater, and a 0.017 in tip. Ambient is 95°F and the material is visibly stiff at the pump. After the first 400 ft² you find dry spray with fingering at the pattern edges, DFT readings of 12–15 mils, and open pinholes along the weld seams. The operator reports 2400 psi at the pump and insists his tip is new. The remaining area to coat is 900 ft².
1. Diagnose why the build is low and the atomization poor. Support your answer with a material calculation for the remaining 900 ft².
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Diagnosis:
- A 0.017 in tip is far too small to deliver 100% solids epoxy at 20–25 mils at a productive rate. Attempting to keep up, the operator speeds up his pass, and film build drops.
- With 150 ft of 3/8 in hose and no heat, pressure drop between pump and gun is severe. 2400 psi at the pump is meaningless - what matters is pressure at the gun. Low gun pressure plus high viscosity equals poor atomization: dry spray, fingering, and a pebbled, under-built film.
- Pinholes at welds are a separate but predictable defect: weld profiles, spatter and pits are hard to coat and often need a stripe coat, plus any air trapped in the pitted weld blows through the film.
Material calculation:
- Theoretical coverage at 1 mil = 1,604 ft²/gal. At 22 mils: 1,604 ÷ 22 ≈ 73 ft²/gal.
- 900 ft² ÷ 73 ft²/gal ≈ 12.3 gal minimum applied film, before losses.
- At a realistic 65% transfer efficiency for airless spray: 12.3 ÷ 0.65 ≈ 19 gal of mixed material.
If the crew is spraying noticeably less than ~19 gal over that 900 ft², the film mathematically cannot average 22 mils - the low DFT is explained before anyone blames the gauge.
2. What equipment and setting changes do you require, and what do you verify on the plural-component system before restarting?
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Changes:
- Tip size: move to 0.021–0.031 in, sized so the gun can deliver the required flow at a pressure that atomizes properly. Confirm with a gun-mounted pressure gauge, not the pump gauge.
- Hose/pressure: shorten the run or go to 1/2 in ID hose to cut pressure drop; raise pump pressure as needed after the tip change.
- Heat: add an inline heater and/or heated hose to bring the material into the manufacturer's recommended application viscosity range (commonly around 100–120°F for 100% solids epoxy). At 95°F ambient the material may still be too stiff, and heat also shortens pot life - recalculate batch/hose residence time.
- Filters: check and replace gun and manifold filters; a partially plugged filter mimics low pressure.
- Tip wear: "new" should be measurable - check orifice and fan with a tip gauge; a worn tip loses pattern and flow.
- Stripe coat welds by brush before spraying, and inspect weld prep for spatter and pits.
Plural-component verification before restart:
- Volumetric ratio check on both sides to confirm the 2:1 ratio is actually being delivered - off-ratio 100% solids epoxy may never cure properly, which no DFT reading will reveal.
- Inspect pumps, check valves, and purge/cross-contamination between A and B sides.
- Confirm pot life at the actual material temperature and the maximum hose residence time.
- Spray a test panel, measure wet film with a comb and DFT after cure, and check for pinholes before releasing the tank.
3. What is your disposition of the 400 ft² already coated, and what repair do you require?
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Reject the 400 ft².
- Under PA 2, 80% of the 20 mil minimum is 16 mils. Spot averages of 12–15 mils fall below that floor outright - this is not a borderline area-average argument, individual spots fail.
- Pinholes along weld seams are holidays and, in a potable water lining, are paths for corrosion and a sanitary failure.
Repair:
- Verify cure and adhesion of the existing film on a test area - off-ratio or poorly atomized material may be soft or weakly bonded, in which case it must be removed, not built over.
- Abrade or brush-blast the entire rejected area to break gloss and provide a mechanical key (and to remove any amine blush if the recoat window is exceeded), then solvent-clean per SSPC-SP 1.
- Stripe/brush-fill the weld seams and pinholes, working material into the pits.
- Apply build coat(s) to bring total DFT into 20–25 mils, measured wet as you go and verified dry.
- Re-verify with a full DFT survey per the specification and a high-voltage holiday test on the repaired area, plus adhesion spot checks.
- Record the nonconformance, the repair procedure, all readings and the final acceptance - and confirm the full tank, not just the repair, gets the required DFT and holiday survey, because the same rig sprayed everything.