The case for in-house is control and availability
The people are already there. They know the aircraft, they know where the tow bar lives, and nothing has to be scheduled through a third party. On a Friday evening with a Saturday departure, that is worth a great deal. Nobody should talk you out of an in-house crew that is trained, equipped and doing the work often enough to stay good at it. The argument against in-house is almost never about effort or care. It comes down to three things: chemical control, technique currency, and insurance.
Chemical control is a discipline, not a preference
Someone has to know which products are qualified for the surfaces they touch, and has to keep the unqualified ones off the aircraft. Exterior cleaners qualified to AMS 1526 are tested not to corrode aluminium, magnesium and steel alloys, and not to adversely affect acrylic glazing or painted and unpainted surfaces. Boeing's D6-17487 battery goes further: sandwich corrosion, immersion corrosion, acrylic crazing, paint softening, and a 150-hour hydrogen embrittlement test to ASTM F519 on notched cadmium-plated specimens held at 45 per cent load. A consumer product cannot have passed a test it was never submitted to. Inside the cabin the traps are sharper still. Gulfstream's own completion center maintenance handbook runs separate procedures for wood, granite, Lexan, vinyl laminate, plating, stainless steel, leather, vinyl, rubber matting, carpet and fabric, and warns that a silicone-based polish leaves a residue on urethane wood finish that can prevent later repair of that woodwork. The same chapter tells you to clean the urethane before every third polish application so the polish does not build up. That is the level of detail the manufacturer thinks a cabin needs.
Technique spends a film you cannot put back
Aerospace clearcoat builds to roughly two to two and a half mils of dry film. Every correction takes a little of it, and it cannot be replaced without a strip and repaint, which is five to seven weeks out of service on a typical full respray. Polished bare metal is the same problem in another form: the Alclad layer shielding the copper-bearing alloy beneath it is only two to five per cent of the sheet thickness, and repeated aggressive polishing goes through it. Polishing also removes the oxide film that protects the surface, so a polished airframe needs recurring work rather than occasional attention. A technician doing this daily knows how much is enough. A technician doing it twice a year is relearning on your paint.
A wash and a detail are not the same job
This is where most in-house and vendor comparisons go wrong. A wash is a few hours and a controlled product, removing airport film and light soil so the aircraft presents well. A detail is crew-days treating leather, Alcantara, veneer lacquer, stone, plated metal and acrylic as six separate material problems, plus paint correction, brightwork and belly degreasing, carried by insurance at limits an FBO will accept. Your crew doing the first one well says nothing about the second, and the second is not a bigger version of the first. Fire retardancy makes the point: operator guidance is consistent that aggressive cleaning of seat covers and carpet can strip out the fire retardant, and on transport-category aircraft those materials were certified against the flammability standards in 14 CFR 25.853.
Insurance is the part flight departments underweight
A specialist working on your aircraft should carry aviation cover written for work performed on aircraft, including hangarkeepers liability for damage to non-owned aircraft in its care, custody or control. The wording is what matters, not the certificate: specified limits, additional insured status, waiver of subrogation, and primary and non-contributory. All of that exists so that a mistake becomes somebody else's claim. When your own line technician puts a buffer through a clearcoat, none of it applies. The aircraft is yours, the damage is yours, and the repaint comes out of your budget. What a vendor never takes off you is responsibility for the airframe itself: FAA AC 43-4B leaves corrosion inspection frequency and treatment with the operator, under the manufacturer's recommendations or your own program, whoever is holding the cloth.
The failure mode runs in both directions
Choose wrong towards the vendor and you pay a quote, and a scheduling delay, for work your own crew could have done between trips, while losing availability you were already paying for. Choose wrong towards in-house and the failures are quieter and far more expensive: haze burned into a clearcoat that then has to be lived with or repainted, a coating laid over contaminated paint that fails unevenly within a season, cabin wood that can no longer be refinished properly, carpet and seat covers whose fire retardancy has been cleaned out of them. Belly soils are the slowest version of the same thing, since accumulated hydraulic fluid, fuel and exhaust soot hold corrosive material against structure. The second set does not announce itself until it is no longer fixable.
What changes the answer
Volume first. A crew washing several aircraft a week stays current; a crew washing one aircraft monthly does not. Then hangar access, because correction and coating need controlled conditions and unbroken hours. Then the environment: manufacturers recommend strip and repaint at five to six years, and salt-laden coastal air pulls that nearer four, so a Savannah or St Simons based aircraft has less margin for a wash regime that is not quite right. Then the calendar. If a sale, a pre-buy or a repaint decision sits inside the next year, the work has stopped being routine care and belongs with people who do it every day. And if your broker cannot tell you what happens when one of your own technicians damages the paint, that is the answer.