Narrow Fan and Low Film Thickness During Airless Spraying of High-Solids Epoxy
An applicator is spraying an 85 % volume-solids epoxy at 250–350 µm DFT per coat onto a large exterior steel structure. Equipment: 45:1 airless pump, 30 m of 9.5 mm hose, 0.017 in (0.43 mm) tip, fluid pressure at the pump 160 bar. Ambient is 34 °C; product viscosity at that temperature is roughly double the data-sheet value at 25 °C. The fan has narrowed noticeably and heavy dry spray is drifting onto already-coated steel. DFT in the dry-spray zones reads 170–210 µm; elsewhere 320 µm. Pot life is 4 hours at 25 °C. The batch was mixed 3 hours ago, and the pump strainer has not been cleaned since the morning. The applicator proposes adding 10 % xylene "just to get the fan to open up"; the data sheet permits at most 5 % of the supplied thinner.
1. Diagnose the narrowed fan and the low DFT. Rank the corrective actions by how much risk each carries.
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Several causes point the same way, and the ranking matters more than the list.
Lowest risk, do first: clean the pump strainer. A partly blinded strainer starves the fluid supply and produces exactly this pattern of falling pressure and a collapsing fan. Confirm fluid pressure at the gun, not at the pump - 30 m of hose and 160 bar at the pump can leave far less at the nozzle.
Low risk: inspect the tip for wear. A 0.017 in tip worn oversize drops pressure and narrows the usable pattern; a replacement takes minutes and is the cheapest diagnostic on site.
Moderate risk: the batch itself. Four-hour pot life at 25 °C in a 34 °C environment is materially shorter - realistically nearer two hours - and this batch is three hours old. Rising viscosity from an advancing reaction explains both the narrow fan and the poor atomisation that produces dry spray. Nozzle size, hose length and heating can be adjusted, but none of them fixes gelled material.
Ranking: strainer, tip, fresh batch, then equipment changes (shorter hose, heated supply, larger tip at correct pressure). Thinning is ranked last because it trades a temporary spray improvement against the film's long-term performance.
2. The applicator wants 10 % xylene. Assess that proposal against this specific product and service, and state the alternatives you would authorise.
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Reject it. Three separate grounds:
- The data sheet caps addition at 5 % of the supplied thinner. Doubling the cap is outside the manufacturer's tested formulation, and any adhesion or performance claim lapses.
- Solvent added to an 85 % solids epoxy drops viscosity but also reduces sag resistance - expect runs and curtaining on vertical work at 250–350 µm - and increases shrinkage on cure. That shrinkage pulls the film onto the profile peaks and can open pinholes.
- Solvent trapped in a thick high-solids film must escape; at 34 °C the surface skins quickly and the solvent has nowhere to go, leaving pinholes and a weaker film. This structure is specified for exterior exposure with a 15-year durability target; the failure will appear as pinholing and premature breakdown, not in the spray fan.
Authorise instead: mix a fresh batch and keep it cool and shaded; shorten the fluid path or increase tip size to 0.019–0.021 in while holding pressure at the nozzle; use a heated supply line or heater at the pump to bring viscosity to the data-sheet range; and move to a smaller, more frequent spray pass so the dry-spray distance stays short. If any thinner is used at all, it is the supplied thinner at no more than 5 %, and only after the mechanical causes are eliminated.
3. What checks would you require on a test section before production resumes?
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Spray-out and pattern check. A sheet of cardboard at production distance and speed shows whether the fan is uniform and whether the tip is worn. This is quick, cheap and it settles the equipment question empirically rather than by argument.
Wet-film and DFT measurement. Wet-film gauge readings during application, converted using the volume-solids figure, to steer the pass thickness rather than discover the result afterwards. Then DFT mapping on the cured section.
Visual acceptance. Dry spray, orange peel, runs and pinholes assessed against the specification's visual standard on the test section, at the same distance and angle as production work.
Pinhole/holiday check on the cured test section, since solvent entrapment and high-build shrinkage are the specific risks here, not just appearance.
Hold point. Production resumes only when the test section meets DFT and visual criteria. Record the equipment configuration that passed - tip size, pressure at the gun, hose length, batch temperature - so the approved setup is repeatable and any later change is a deliberate decision rather than drift.