What deferring exterior care actually costs

The cost of stretching a wash interval is not the wash you skipped. It is that contaminants which would have rinsed off in minutes stay on the surface through every day and night cycle in between, and some of what happens in that time cannot be reversed by washing later.

Here is the mechanism, from the documents, and here is the part where we stop and tell you what nobody can source.

Nobody will give you an interval, and that is honest

FAA Advisory Circular 43-4B names washing as an element of corrosion prevention and declines to state a frequency. Paragraph 1.8.2 opens with the requirement that operators “must follow a constant cycle of cleaning, inspection, operational preservation, and lubrication to prevent corrosion,” and the list that follows includes “aircraft washing at regularly scheduled intervals.” Regularly scheduled, not every N days. The AC is explicit that frequency remains the operator’s responsibility and that its procedures are an acceptable means rather than the only one.

Paragraph 1.8.1 is where it tells you why: aircraft exposed to salt air, heavy industrial pollution or over-water operations need a more stringent program than one in a dry environment.

For contrast, somebody does publish a number. NAVAIR 01-1A-509-2, the Navy’s cleaning and corrosion control manual for aircraft, paragraph 2-3, requires Navy aircraft to be cleaned at least every 7 days aboard ship and at least every 14 days ashore where the aircraft specific manual says nothing else. The same paragraph lists conditions requiring more frequent cleaning, including exhaust soils accumulating in impingement areas and exposure to salt spray. Paragraph 2-3.2 goes further and names conditions for cleaning immediately, including apparent salt deposits and exposure to significant salt water.

That is a fleet operating in the harshest salt environment there is, setting 7 and 14 days for itself. It is not a civil requirement and your aircraft is not an F/A-18. It is a useful calibration point when somebody tells you twice a year is plenty.

Why deferral compounds rather than accumulates

The single most useful sentence we found is in NAVAIR 01-1A-509-1, paragraph 3-13.1.1: “Condensed moisture usually evaporates as the surrounding air warms, but leaves contaminants, including salts, behind.”

Read that again with a parked aircraft in mind. Every night the surface goes below dewpoint and wets. Every morning it dries. The water leaves. The salt does not. Paragraph 3-8.8 of the same manual notes that nightly temperature drops cause greater condensation and lead to increased corrosion rates.

And a wet surface with contaminants on it is a better battery than a wet surface without them. AC 43-4B paragraph 2.4.4: “Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing electrolyte conductivity and also the rate of corrosion.”

So the aircraft that has not been washed since March has not had one long dirty period. It has had roughly a hundred and fifty wet and dry cycles with the contaminant concentration rising each time.

Filiform corrosion, and a disagreement worth knowing about

Filiform is the one that happens under the paint. AC 43-4B paragraph 2.5.7 describes it as recognisable by “a worm-like trace of corrosion products beneath a paint film,” occurring when relative humidity sits between 78 and 90 percent and the metal surface is slightly acidic. It starts at breaks in the coating and travels underneath by diffusion of water vapour and oxygen through the film. The FAA lists washing the aircraft to remove acidic contaminants as one of three prevention measures, alongside storing below 70 percent humidity and using low diffusion coatings.

Here the primary sources disagree and we would rather show you that than pick the scarier one. NAVAIR 01-1A-509-1 paragraph 3-9.7 puts the humidity band at 65 to 90 percent and states that filiform “only affects surface appearance, but does not weaken or destroy the base metal.” The FAA says it can lead to intergranular corrosion, especially around fasteners and at seams, if it is not removed and treated. Two government documents, same phenomenon, different severity. Neither mentions chloride as a filiform driver, incidentally, so if you see salt named as the cause of filiform specifically, that is somebody blending two mechanisms.

Exhaust deposits are a different problem from dirt

AC 43-4B paragraph 4.2 is blunt: “Both jet and reciprocating engine exhaust gas deposits are very corrosive.” Paragraph 4.2.1 adds the part that matters operationally, that gaps, seams, hinges and fairings trap exhaust deposits where normal cleaning methods cannot reach.

Navy doctrine classifies exhaust deposits as a heavy soil alongside carbonised oil and aged preservatives, calling for dedicated solvent based cleaners rather than general aircraft wash. That is the real reason belly work is priced differently. It is a different soil requiring a different chemistry and more hand time, not a general wash with more effort behind it.

You will read that soot bakes on, or that it etches the topcoat. We looked for a source for both and did not find one, in the FAA material, in either NAVAIR volume or anywhere else authoritative. The documented version is the access problem, and it is enough.

The part that genuinely does not come back

PPG specifies its Desothane HS CA8000 polyurethane topcoat, which meets SAE AMS 3095 among other qualifications, at “50 to 75 microns (2.0 to 3.0 mils)” dry film. That matches the Jet Glo figure and it is the band most high gloss aircraft systems live in. Not all of them: AkzoNobel’s Aerodur ARC is specified at 125 to 150 microns, so check what is actually on your airframe rather than assuming.

Correction removes material. That is what it is. Every pass takes some of a film that started around the thickness of a human hair and is never replaced until the aircraft is repainted. So an oxidised finish that needs compounding is spending something that does not regenerate, and a finish that was washed before it oxidised is not.

No manufacturer publishes a number of permissible passes or a minimum remaining film thickness. We checked PPG, AkzoNobel, Sherwin-Williams and the Navy manuals. If anyone quotes you one, ask which document it is in. The arithmetic you can do yourself off the film thickness figure is the honest version of this argument, and it is a stronger one than a made up number.

What the finish was built to survive, and what it was not

MIL-PRF-85285E, the polyurethane topcoat specification, gives a sense of the envelope these coatings are qualified against. The coating must show no cracking when bent over a mandrel at minus 51 degrees Celsius. It must survive an hour at 121 degrees. It must sit 30 days in a humidity cabinet at 49 degrees and 100 percent relative humidity without blistering, softening or loss of adhesion.

These finishes are engineered to take a beating from temperature and moisture. What they are not engineered to do is sit under a concentrating layer of salt and exhaust for six months. That part is on the operator, which is exactly what AC 43-4B says.