Hollow-Metal Doors and Frames: Orange Peel, Dry Spray, and Sags in One Shift
Your crew is refinishing 32 hollow-metal door slabs and their frames in a school multipurpose room over summer break. The specification requires a waterborne acrylic semi-gloss at 3.0 to 5.0 mils dry film thickness over the factory primer; bare or damaged areas are field-primed with a direct-to-metal primer. You are spraying with an airless unit fitted with a 0.015 in reversible tip, running 3,000 psi, indoors at 88°F and 30% relative humidity, with two box fans running for air movement. After the first pass, the door faces show a fine, uniform pebbly texture; the frames show a sandy, dusty film that rubs off on a finger; and four doors have heavy sags along the bottom rail. An electronic film gauge reads 1.6 to 1.9 mils on the frames and about 5.5 mils on the door faces. The general contractor needs the doors hung and hardware reinstalled in two days.
1. Diagnose each of the three defects separately and name the specific parameter you would change for each.
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Orange peel on the door faces (about 5.5 mils): the film is heavy and atomization is coarse, so it cannot level. Check the tip first - a worn 0.015 in orifice behaves like a much larger one, dropping pressure at the tip and producing a coarse pattern. Then: replace or step down the tip, back off fluid output, hold 10–12 in and perpendicular, overlap about 50%, and stop the box fans - at 88°F and 30% RH the water flashes before the film flows. A manufacturer-approved extender helps open time.
Dry spray on the frames (1.6–1.9 mils): the film is largely partially dried overspray that never coalesced and is below the 3.0 mil spec. Causes are distance too great, moving air, and hot, dry conditions. Kill the fans, spray closer, reduce fluid output, add extender, and consider brush-and-roller on the frames - frames are narrow, high-loss geometry where spraying wastes material and struggles to build film.
Sags on the bottom rail: a heavy pass and gun dwell at a horizontal member, plus spraying down into a rail that collects material. Use lighter passes at consistent speed, spray the rail as its own pass, keep the gun perpendicular, and never tilt it to reach underneath.
These are three different problems; changing only the pressure fixes none of them reliably.
2. The spec is 3.0 to 5.0 mils dry. The product is 45% volume solids. Calculate the wet film required, and explain how you would verify build on these doors and frames.
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Wet film = dry film ÷ volume solids fraction.
- 3.0 ÷ 0.45 = 6.7 mils wet (minimum).
- 5.0 ÷ 0.45 = 11.1 mils wet (maximum).
So the target band is roughly 6.7 to 11.1 mils wet. If you measure about 12 mils wet, you are above the dry maximum and asking for sags, blocking, and poor cure - which is exactly what the 5.5 mil dry reading on the door faces tells you.
Verification: check wet film with a wet film comb immediately after each pass at several points while the film is still wet; you cannot recover a wet reading later. Check dry film with a magnetic/electronic gauge on the steel - calibrate it on a smooth panel of the same substrate, and take readings on flat, smooth areas rather than on the pebbled texture or near edges, which read falsely high. The frames at 1.6–1.9 mils are below spec and need another coat; document the readings by door number so the GC sees objective evidence rather than an opinion.
3. Build the recovery plan that gets all 32 doors and frames to spec and hung in two days.
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Today, remaining shift: stop the fans and correct the settings before spraying anything else - verify or replace the tip, bring the gun to 10–12 in with 50% overlap, add extender per the data sheet, and switch the frames to brush/roller so film can be built in the narrow geometry where spraying loses material. Re-coat within the manufacturer's recoat window; waterborne acrylics typically allow recoating after a few hours but must be recoated before full cure or the surface must be abraded.
Four sagged doors: let them cure enough to sand, block the sags flat without cutting through to primer, feather, then re-coat. Do not try to bury a sag.
Frames: one additional coat by brush/roller to reach 3.0 mils minimum, checked with the gauge.
Sequencing for the two-day deadline: spray door slabs flat on racks in a controlled area so they dry evenly and can be handled; brush frames in place. Reinstall hardware only after the film is hard enough not to block or print. Reserve the last half-day for gauge readings and touch-up, and give the GC a written note if the humidity climbs, since a waterborne film that stays soft overnight will not be ready for hardware.