# Aluminium Skin Blooms Before It Pits Aluminium near the coast goes through a chalky white bloom long before it pits. The bloom wipes off and comes back, then stops wiping off, then stops polishing out. Knowing which of those stages a panel is in tells an owner whether they are buying labour hours or buying metal. ## Stage one leaves powder on a clean cloth Run a clean dry cloth across a polished aluminium panel on a coach that has wintered near the coast and the cloth comes away grey. That grey is aluminium oxide in powder form, and its arrival is the first measurable event in the sequence this post follows. It is not damage. Aluminium forms an oxide layer within seconds of meeting air, and that layer is what protects the metal underneath. On a clean panel the layer is transparent and a few molecules thick. What produces visible powder is that layer being repeatedly disrupted and reformed, each cycle leaving behind loose product rather than a coherent film. Chloride is what does the disrupting. Salt ions attack the passive oxide film at weak points, the film reforms, the ions attack again, and the by product accumulates as bloom. Fresh water rinsing removes the ions and the bloom together, which is why the same panel on a coach kept inland stays bright with almost no attention and the coastal one greys within a season. At this stage the owner has lost nothing except appearance, and the appearance comes straight back with a wipe. ## Aluminium fails white and steel fails brown The colour difference is not cosmetic trivia, it changes what the owner should do. Iron oxide is porous and flakes away, exposing fresh steel underneath, so rust is self perpetuating and has to be cut out. Aluminium oxide is dense and adherent, and left alone it forms a protective barrier that slows further attack. An untreated aluminium panel does not consume itself the way a steel one does. It reaches a chalky grey equilibrium and largely stops. That is why an owner has a genuine choice that a steel owner does not. Letting aluminium go grey is a defensible decision. Plenty of working trailers and older coaches live out their service lives in exactly that state with no structural consequence. Polishing is a cosmetic programme, not a preservation programme, and anybody who tells an owner that polishing is protecting their coach has the mechanism backwards. What polishing does is remove the oxide, which briefly leaves the bare metal more exposed until the film reforms. The exception is where chloride concentrates rather than sitting evenly. Under a trim strip, behind a rub rail, in the crevice at a lap joint, around a fastener, in the tide mark at the bottom of a panel where run off collects. In those locations the attack does not reach equilibrium because fresh chloride keeps arriving and never rinses away, and that is where uniform bloom turns into localised pitting. ## Four stages, and how a panel reports which one it is in A single sidewall usually contains more than one stage at once, because the bottom third takes road spray and collects run off while the top gets rinsed by every rain event that lands on the roof. Reading the coach panel by panel is therefore less useful than reading it zone by zone, and an owner who concludes the whole trailer is at one stage has almost certainly averaged three different conditions into one wrong answer. The four stages below are progressive and a zone does not skip any of them, so the earlier ones are always the cheaper place to intervene. - Loose bloom that transfers to a dry cloth and leaves the surface bright underneath, which is cosmetic and reversible with a rinse - A dull grey film that a dry cloth will not shift but a mild polish removes in one pass, which is still cosmetic and costs an hour or two of bench time - Grey that returns within weeks of polishing, or that only comes off with an aggressive compound, which means the surface is now microscopically rough and holding product in the texture - Visible pitting, small dark points that a fingernail catches and that stay dark no matter how long the polish runs, which is metal loss and is not a finish problem any more ## Finding the line on your own coach with two hands The test that settles which stage a zone is in takes five minutes and needs a dry cloth, a small amount of mild polish and reasonable light. Pick a section about the size of a dinner plate low on a sidewall, where the condition is usually worst, and wipe it with the dry cloth first. If the cloth greys and the metal brightens behind it, that zone is at stage one and nothing further is needed. If the dry cloth does nothing, work the same spot with polish on a cloth by hand for a minute and watch what comes up. Stage two comes up bright and even across the whole patch. Stage three comes up patchy, with a background haze that will not clear no matter how long you keep at it. Then run a fingernail slowly across the polished area, listening rather than looking. A stage two surface is silent and slick. A stage three surface has a faint drag. A pitted surface catches audibly, and if you tilt the panel to the light the pits show as dark points that survive polishing and are not reflecting. That catch is the line. Everything before it is labour and everything after it is metal, and the distinction matters more to the bill than any other single observation an owner can make. Repeat the test in three zones on the same wall, one low, one at mid height and one near the roof line, and write down the answer for each. Most coastal coaches return a different answer in each zone, and knowing that in advance is what prevents an owner from authorising a full polish and then being surprised when the bottom band refuses to come up and match the rest. The same test on the front cap and on a compartment door is worth doing too, since those surfaces are often painted rather than bare and behave differently again. ## The point at which more hours stop buying more shine Polishing aluminium is a progressive cutting operation. Each stage of compound removes a layer of metal along with the oxide, then the next finer stage removes the scratch pattern the previous one left. On a surface at stage two, that sequence is short and the result is even. On a surface at stage three, the first cut has to go deeper to get below the roughened layer, which means more passes, more compound and more hours before the finishing stages even begin. The finish is achievable. It just costs a multiple of what the same panel would have cost a year earlier. Detail labour posts at $95 per hour, and paint correction and polish runs $500 to $4,000 depending on surface area and the state it starts in. Full interior and exterior detail runs $500 to $3,500. Those bands are wide for a reason and the position within them is set almost entirely by the stage the metal is in, not by the size of the coach. A large trailer at stage two can cost less than a small one at stage three. Past the pitting line the arithmetic inverts and honesty requires saying so. Polishing a pitted panel makes the surface around the pits brighter, which raises the contrast and makes the pits more obvious rather than less. Owners regularly pay for a full polish on a pitted lower band and are unhappy with the result, not because the work was poor but because the work made a defect legible that was previously camouflaged by uniform dullness. When the fingernail catches, the useful conversation is about accepting the patina or about replacing the panel, and OCRV Center will say that before quoting hours rather than after. ## Clear coating polished aluminium and what it costs later The obvious solution to an annual polish is to seal the metal so it stops oxidising, and clear coats sold for that purpose do work initially. The difficulty is the failure mode. A clear film over polished aluminium fails in patches rather than uniformly, usually starting where the film is thinnest at an edge or a rivet line, and moisture that gets under the film sits against bare metal with no air exchange and no ability to rinse. Filiform corrosion runs along under the coating from that break in fine threadlike lines, and the panel now has dull patches under a glossy film that cannot be polished because the polish cannot reach the metal. Removing a failed clear coat from a full sidewall is more hours than several years of straightforward polishing would have been, and the surface underneath is rarely uniform when it comes off. That is not an argument against ever coating aluminium. It is an argument for knowing that the decision is close to permanent, that it commits the owner to maintaining the film rather than the metal, and that the total labour over ten years is usually higher rather than lower. Owners who like the look and are willing to keep after the film do fine. Owners who coat specifically to stop doing maintenance generally end up doing more. Where the panel has already gone past cosmetic recovery, replacement is the honest path and it is a structural job rather than a finish job. Aluminium skin and sidewall replacement runs $2,500 to $25,000, and what determines the position in that band is how many panels come off, whether the framing behind them is sound and whether the coach is a riveted monocoque shell or a framed body with a skin over it. ## Related - [/services/collision-structural/aluminium-skin-replacement/](https://ocrv.guru/services/collision-structural/aluminium-skin-replacement/) - [/services/paint-body/paint-correction-polishing/](https://ocrv.guru/services/paint-body/paint-correction-polishing/) - [/vehicles/rvs-motorhomes/vintage-classic-rv/](https://ocrv.guru/vehicles/rvs-motorhomes/vintage-classic-rv/) --- Source: https://ocrv.guru/blog/coastal-ownership/aluminium-skin-blooms-before-it-pits/ OCRV Center, 23281 La Palma Ave, Yorba Linda, CA 92887. (949) 799-3387. Serving Laguna Niguel, California. All work performed in shop. No mobile or roadside service. Licences: California Bureau of Automotive Repair ARD00288521; EPA CAL000367879. Published ranges are ranges, not quotes. Every figure traces to https://ocrv.guru/prices/.