Old aluminum mast repaint / keep raw

Discussion in 'Metal Boat Building' started by xslim, Jul 12, 2016.

  1. xslim
    Joined: Jul 2016
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    xslim New Member

    Hi All!

    I'm rebuilding the 30-year old aluminum sailboat.

    I'm hoping at the end of rebuild / refit to have a low maintenance go-anywhere sailboat, so I don't care if it's not pretty.

    The paint on the mast and topsides is chipping off, so I was thinking, is it OK to strip the paint completely and leave the mast unpainted (raw) ? The mast is deck stepped.

    What would be the drawbacks?

    Also, can anyone recommend best way to lead cables from mast to cabin? Currently they are lead thru plastic ring and sealed with some sort of sikaflex / silicone.

    Thank you!
     
  2. Barra
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    Barra Junior Member

    No Paint.

    My unpainted un -anodised 15 meter mast is now 17 years old. Lived in a Saltwater tropical enviroment. Its still immaculate. I may rebed the fittings in a few years (20 year birthday party maybe) Never been coated with anything.

    Paint application will only hasten corrosion. Apply fittings with liberal amounts of barium chromate.
     
  3. xslim
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    xslim New Member

    Thank you for reply!

    I newer heard of barium chromate.
    I know of teflon paste and sikaflex.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Aluminum masts have been painted successfully for many decades and it offers good durability. The addition of paint does not increase the probability of corrosion, in fact the exact opposite can be expected.

    What Barra is mentioning (my assumption) is Duralac (barium chromate), which is a paste and used on dissimilar materials where corrosion might be expected. This isolation membrane can be accomplished a few other ways, though most things typically attached to an aluminum spar (stainless) really don't need it. Technically, stainless and aluminum should be isolated, but in reality, you can rivet or screw a stainless track (or whatever) to an aluminum extrusion and it'll hold for 30 or 40 years. In this time you will get some pitting, depending on maintenance, but in all honesty, how much durability do you really need.

    As to passing lines or wires through a cabin roof, you can use one of several approaches. The bullet proof way is a bulkhead fitting, which is like a mechanical compression fitting and it has an internal rubber, that squeezes the wire bundle to prevent leaks. Something as simple as a rubber boot will do, as can a healthy bead of polyurethane. The low dollar route is to side a rubber fuel line over the wires, with enough length to pass through. Drill a hole slightly smaller than the rubber hose and force the hose into the hole and seal with polyurethane. Not pretty, but it works well. I use bulkhead connectors myself, as they look professional and can be stepped on, without concern.
     
  5. xslim
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    xslim New Member

    Painting the mast - here the question is - if I can strip it to bare aluminum and skip on painting - I'll do that.

    For passing the wires - do you use bulkhead fitting for each wire? So as a minimum of 3 small holes - VHF, light and windsensor ?

    I know Duralac. I use Tef-gel. Almost same. I have few parts that previous owner skipped on using it. Now those screws are seized into aluminum and I needed to drill them out.
     
  6. Barra
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    Barra Junior Member

    Aluminium dingys are very popular in Austalia. Very few are painted.

    The French have aluminium production yachts with unpainted hulls.

    Many comercial aluminium boats that don't mind the asthetics do not paint what they don't have to. (antifoul/nonskid excepted)

    Any corrosion under the paint pits the aluminium terribly ( I understand the chemicals involved in the corrosion can't escape due to the paint, making the corrosion at that point worse) especially near mast fittings. Go for a walk around any marina ignoreing the marina queens that never get used and hence the coating never gets compromised for a look at what paint does to a mast. In the real world where PAR obviously doesn't reside, if you are happy with the look then keep paint well away from an aluminium mast.

    Ps. stripped my 12month old pwder coated aluminium anchor winch 15 years ago as the coating started to fail through every day wear and tear (blisters of corrosion). Haven't had to touch it since:D
     
  7. rwatson
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    rwatson Senior Member

    You mean like this picture

    "The paint works well until it is breached, and water enters the rift. This breach could be caused by a fastener, an antenna base, a navigation light, a hinge, a lockset, a nick, or a scratch—and it could be microscopic. Initially the wound is self-healing: The oxide coating forms, but instead of protecting the aluminum, it lifts the edge of the paint and allows water to migrate farther under the coating. At that point, the chemical equation changes, as moisture and aluminum, in the absence of air, are the ideal incubators for a phenomenon known as poultice corrosion."

    http://www.proboat.com/2014/12/keeping-paint-on-aluminum/
     

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    Last edited: Jul 13, 2016
  8. PAR
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    PAR Yacht Designer/Builder

    99% of aluminum hulled yachts are painted, damn I guess they'll all sink soon. 99% of all aluminum built aircraft are painted, damn they must be falling from the skies like rain.

    Barra, we don't know your experience, nor expertise, but clearly a misrepresentation of coatings on aluminium isn't very helpful or particularly accurate.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    The image shown is not the result of poultice corrosion. For poultice corrosion to occur it needs a wet wick like substance such as foam/wood etc in direct contact with the aluminium.

    Unpainted ally is perfectly fine. Many high speed ferries in the early mid 90s are all unpainted between the hulls of cats. Also Link spans and many other structures which are shore side too. As are many fishing and sports boats in Western Australia are all unpainted. Ally, the correct grade of ally, has no issues with being left exposed. The 6000 series is a bit more susceptible, but so long as no dissimilar metals are present, this minimises the issue. Extrusion that have copper content above 0.10% are the ones to be careful of as well as over strain-harden tempers too, and thus to be used with caution and wisely.
     
  10. rwatson
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    rwatson Senior Member

    Besides the fact that planes don't fly in salt water ( and DO suffer corrosion as any LAME will tell you) - did you read the linked article ?
     
  11. rwatson
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    rwatson Senior Member

    Which is what it is.

    "water is trapped by debris and dust, thus becoming acidic. The acidic solution is called aluminum hydroxide, which perforates the window frame. This leaves white deposits on the bare and unprotected surface of the metal. The retention of moisture makes the corrosion continue, even when surfaces are dry. "

    https://www.corrosionpedia.com/definition/918/poultice-corrosion
     
  12. Petros
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    Petros Senior Member

    I do not know if it is 99 percent, but certainly American Airlines, Fed Ex, and many other commercial carriers do not paint their aircraft, most military aircraft are not painted either.

    Most aluminum canoes, row boats and skiffs come unpainted as well.

    I suspect much of it depends on the alloy and the environment. raw aluminum is corrosion prone, the higher the strength generaly the more prone it is to corrosion. Most Al alloy comes with a thin, almost clear coat of oxide to protect the underlying metal. bare aluminum is shinny, that is not the case with an aluminum mast.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Nope, it is not.

    It can be easily explained why, using a Pourbaix diagram of aluminium. This is shown below:

    Pourbaix diag of ally.jpg

    Crevice corrosion can occur in narrow ‘gaps’ or ‘recesses’ of metal to metal contact and metal to non-metal where the water is not refreshed, i.e. stagnant and, where an environment can develop in the crevice that is different from the surroundings. Additionally aluminium is sensitive to pitting corrosion in environments where the pH is close to neutral and so when there is a score mark from manufacturing or a mark left by a screw/thread, these are termed events that locally attack the otherwise stable oxide film.

    Where you get the development of localised environments in the score mark or pit (acid) or crevices, you also get a localised concentration of the now aggressive ions ( e.g. chlorides) which increases this susceptibility to crevice (and pitting) corrosion and as shown by the Pourbaix diagram increases with the increasing chloride content.

    This is not the same process as Poultice corrosion. Despite the end result being the same. Just as this is not exfoliation corrosion, yet the end result is the same.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Nonsense.

    Aluminium is protected by a film of Al2O3 or Al2O3.H2O of various thicknesses, though it is nominally around 50 Angstrom thick. It is this protective film that provides aluminium with its characteristic resistance to corrosion despite your opine to the contrary.

    This film forms rapidly once exposed to oxygen. Additionally, if the surface of aluminium is scratched sufficiently to remove the oxide film as noted above, a new film quickly reforms, in most environments which is why it is very corrosion resistant. But when prevented from forming naturally, is another matter. As a general rule, the protective film is stable in aqueous solutions of the pH range 4.5-8.5 see the Pourbaix diagram above, whereas it is soluble in strong acid or alkalis, leading to rapid attack of the aluminium as noted by the picture by RWatson above.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Most all military aircraft are painted, not the other way around. In fact, show me the current operational military aircraft that aren't painted. Most all commercial aircraft are painted, not the other way around, Ditto aluminum yachts. I'm not arguing for or against coatings on aluminum, though am suggesting a lot more aluminum structures are coated, rather than left raw. Personally, I don't care either way, owning both on many boats, devices, etc. On things I handle often, I prefer a coating, if only to keep the oxide from getting all over everything.

    As to aircraft not being particularly exposed to salt water environments, well this just isn't reasonable, nor logical, considering from where they fly.

    [​IMG]

    Must be a special white alloy they're using.

    [​IMG]

    You don't think this is raw aluminum do you?

    [​IMG]

    Maybe this image will clear things up a bit. Even those aircraft that are seemingly polished aluminum are coated, otherwise they're buffing their brains out every month, just to keep it pretty.
     
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