What wildfire ash does to a finish, and what not to do about it
Wildfire ash is alkaline and it is abrasive, and the single most expensive thing anyone does about it is wipe it off dry. The right first move is to flood the surface with clean water and carry the ash off the finish rather than dragging it across the finish.
We work Van Nuys, Burbank and Orange County, and this comes up every year. The aircraft that end up needing correction are almost never the ones that sat with ash on them for a day. They are the ones somebody helpfully wiped down.
What ash actually is
Wildfire ash is the mineral residue left after combustion, mostly oxides and carbonates of calcium, potassium and magnesium, mixed with unburned carbon and whatever the fire went through. Two properties matter.
It is alkaline. When ash gets wet, and it will, from dew, from marine layer, from a rain shower, the water becomes a caustic solution sitting on your topcoat and on the polished metal. On brightwork this shows as dulling and staining. On paint it is less dramatic but it is not nothing, particularly if it sits.
It is hard and irregular. Under magnification, ash particles are angular. That is the whole problem. A microfibre towel dragged across a dry, ash-covered surface is a lapping process. You are not removing the ash. You are using it to sand the aircraft.
Why the dry wipe is the mistake
Think about where the four cleaning variables sit. Somebody looking at a dusty aircraft has no water and no chemical available, so all they have left is agitation, and agitation is the one variable an aircraft finish caps hardest. A dry wipe is pure agitation with abrasive media loaded into the towel.
The marks it leaves are fine and directional, and in a hangar they are almost invisible. In direct sun on the ramp they show as haze and swirl, and correcting them means removing topcoat that does not come back. On a finish that is 50 to 75 microns to begin with, that is a real cost.
The same logic applies to a leaf blower, which is the other instinct. Air moves the lightest particles and leaves the heavy angular ones sitting there, and then somebody wipes.
What to do instead
- Flood first, with clean water and low pressure. The goal is to float the particles off, not to blast them. High pressure on a heavily contaminated surface drives grit into the finish and into seams.
- Do not touch the surface until the ash is gone. No mitt, no towel, no hand. If you can still see or feel grit, keep rinsing.
- Wash top down with plenty of lubrication. Once the loose material is off, a normal wash with an aviation cleaner and a lot of solution is safe. Change your water often, because the ash is now in your bucket.
- Pay attention to the transparencies. Windscreens and cabin windows are acrylic. They are softer than the paint and they are the most expensive thing on the aircraft to mar. Flood them, never dry wipe them, and keep glass cleaner away from them. The FAA’s own guidance in AC 43.13-1B tells maintainers not to use window cleaning sprays, alcohol, acetone or lacquer thinner on transparent plastics because they cause crazing.
- Rinse the areas you cannot see. Ash collects in the same places water does: control surface hinge lines, flap tracks, gear wells, the belly aft of the wing root. Alkaline residue sitting in a lap joint is a corrosion conversation, not an appearance one.
- Dry it. Standing water plus alkaline residue is worse than either alone.
Brightwork and unpainted metal are the urgent part
If the aircraft has polished leading edges or unpainted panels, that is where alkaline residue does visible harm fastest. Painted surfaces have a coating between the residue and the metal. Polished aluminum does not. Get the ash off bare metal first, and if there is already staining, that is a polishing conversation rather than a washing one. See brightwork polishing for what the difference costs.
If the aircraft has been sitting in it
Ash that has been through a wet and dry cycle is bonded, not loose, and it will not rinse off. At that point you are into a chemical decontamination step rather than a wash, and the products and dwell times matter. This is worth handing to somebody who does it rather than experimenting, because the failure mode is exactly the one described above.
We have crews at Van Nuys, Burbank and Orange County, and during a fire event the useful thing we can do is get to the aircraft before somebody with a towel does. The exterior service is the same service it always is. It is the sequencing that changes.
The one thing worth saying to a crew
If there is ash on the aircraft and you have no water, do nothing. Waiting costs you nothing and a dry wipe costs you topcoat. That instruction, given once to line crews before fire season, has saved more finishes than any product we have ever used.
Questions people actually ask
Short answers
Should you wipe wildfire ash off an aircraft?
No. Ash particles are hard and angular, so wiping a dry, ash-covered surface laps the abrasive across the finish and leaves fine scratching that requires correction to remove. Flood the surface with clean water first and do not let anything touch it until the ash is gone.
Is wildfire ash corrosive to aircraft?
Ash is alkaline, and once it gets wet from dew, marine layer or rain it becomes a caustic solution sitting on the surface. On painted areas that is mostly a staining concern. On polished or unpainted metal, and in seams where it collects, it is a corrosion concern.
How do you clean ash off an aircraft properly?
Flood with clean water at low pressure to float the particles off, avoid any contact until the grit is gone, then wash normally with plenty of solution and frequently changed water. Rinse the areas that collect debris, including hinge lines, flap tracks and gear wells, and dry the aircraft afterwards.