Years ago when I was still a young sprog engineer, I was confronted by a fairly new Cessna 337 that had been prepared by the owner for re-painting. The aircraft had been used for fish spotting so even though not old, the aircraft had been subjected to the effects of salt water spray and some surface corrosion was the result. The owner decided that a re-paint was the answer, so in the ‘off season’ the process was started. Well being a thorough (and frugal) type of guy, he trucked the aircraft to the local shipyard where he had a mate who said he could give the aircraft a light sand blast. This would not only strip the old paint very easily, but also remove the corrosion. Well the sand blasting certainly did a great job of both. The aircraft emerged with no paint, no corrosion but with very badly damaged skin panels. After much debate the owner finally got me to check it out. The damage was appalling: all of the skins were expanded outward (toward the direction of the blasting) and were peppered with small sharp dents from the sand. Unfortunately when I gave my verdict ‘guilty of aircraft murder’ the owner nearly cried. The aircraft was written off and never flew again.
More recently I have come across an aircraft that had been stripped and re-painted some years previously but had always been hangared since. Evidence of corrosion near lap joints kept appearing during the following years. Each time the spots were cleaned back, primed and the paint touched up. Unfortunately only a short time later more would pop up elsewhere. Eventually the aircraft required to have the wing removed for other unrelated maintenance. It was only then that the true extent of the corrosion damage became evident.
It would appear that during the original re-paint process the owner, thinking he was doing the right thing, had the aircraft skins prepared for painting by applying ‘Alumiprep’, a phosphoric acid based cleaner, brightener and pre-paint conditioner for aluminium. Alumiprep can be used to deep clean and brighten an aluminium surface prior to painting or alodining. This material is excellent when used as intended by the manufacturer, in a controlled environment and when used by personnel who have been properly trained in its use. Unfortunately incorrect use can do irreparable damage.
Alumiprep should be used on unassembled parts only. If it is used on assembled parts such as wing skins with lap joints, capillary action will draw the Alumiprep between the sheets and even between the rivets and the sheets, particularly if the rivets are not really tight in the hole. The Alumiprep data sheet quite clearly states that the material should remain on the aluminium surface for between two and five minutes then washed off. Remember it is phosphoric acid based – it is corrosive! When used on one surface only there is no problem washing it all off. This is not the case where lap joints are involved – it is impossible to wash all of the Alumiprep out as it will have penetrated into even the most minute of crevasses. This means of course that the corrosive material remains in the lap joint forever, continuing to eat away for years. No wonder corrosion reappears a bit further down the track.
The other thing that one must be aware of is that the Alumiprep data sheet includes the warning that ‘Alumiprep should not be used on high copper bearing aluminium alloys’. On a normal Alclad aluminium alloy sheet this is not a problem as we are treating the pure aluminium cladding. The most common aluminium alloy used in light aircraft construction is 2024-T3, an alloy that uses copper as the primary alloying element. As I said, this is not a problem on clad sheet but if the sheet has had some prior surface scratching or corrosion that has penetrated the cladding, the high copper content underlying 2024-T3 alloy will be exposed and thereby open to attack by the Alumiprep solution. Care, knowledge and experience is vital when using this, or similar materials, on aircraft structures. An aircraft can be literally destroyed if it is not treated correctly.
The last point I need to make is that paint stripping, corrosion removal, surface preparation and painting are all ‘aircraft maintenance’ processes as defined in the Civil Aviation Regulations. CAR Schedule 8 allows a qualified pilot to undertake certain maintenance but in relation to painting it only allows ‘application of preservative or protective materials, but only if no disassembly of a primary structure or operating system of the aircraft is involved’. Clearly corrosion assessment, removal, treatment or repair is not covered by Schedule 8 so these activities must be carried out under the auspices of a maintenance organisation approved to carry out such maintenance.
As we can see from the examples quoted, it can be very expensive attempting to ‘save money’ doing some jobs ourselves. Leave it to the experts – people who are properly trained, experienced and approved. This way we save money in the long run, remain safe and we’re still legal.