Old aluminum mast repaint / keep raw

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

  1. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    I dont know what point you are trying to make with pictures of pretty painted planes, that painted airplanes dont corrode ? Certainly they dont as badly as marine gear, but they do corrode. For that reason they have to be painted often.

    Just google "Aircraft aluminium corrosion" in Google

    As for whether painted planes corrode, lets rely on the experts

    "Most repaint their airplanes every four years, often during a scheduled C- or D-check, - Airplanes should never carry more than two layers of paint. With more than two layers, operating efficiency drops, inspections become more difficult, and corrosion can start in chips that remain under a fresh topcoat. . "

    http://www.boeing.com/commercial/aeromagazine/aero_05/textonly/fo01txt.html

    So, corrosion CAN start under paint.

    Its plain to see that raw aluminium has "self protection" built in to a large extent, so why paint it ? Obviously for looks, and aero/hydro-dynamic resons. Aluminium doesnt have a corrosion problem without paint.

    The link I provided has plenty of corrosion examples under paint - if you can take the time to actually look.

    As a final example. take a walk down in the dinghy racks at your local yacht club, that use aluminium masts and look at all the powdery 'divots' in between the remaining paint, where corrosion has taken big bites from under the paint before it peeled off. The unpainted masts have dull and shinier sections, but not powdery little divots.
     

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  2. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    XSlim - this is good reading on the topic. They also call the kind of oxidisation caused by pooling moisture "poultice corrosion" , but I cant be bothered arguing the point.

    "If you get the chance, take a close look at the U. S. Coast Guard’s heavy-weather rescue vessel, the venerable 47-foot Motor Life Boat, or any of its other aluminum response craft. Check out the paint job. What you’ll notice, beyond the familiar Coastie stripe and bottom antifoulant, is that these vessels are all unpainted. And it turns out that this saves the Coast Guard untold sums in paint, metal maintenance, and repair while it increases the service life of the alloy hulls."


    http://www.cruisingworld.com/how/bare-better
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    No one is saying painted aluminium doesn't corrode. The pictures are the same aircraft owners, that were suggested are employing unpainted craft, which is clearly not the case. The same is true of military aircraft, equipment and ships too. The USCG has made some exceptions, but generally coated surfaces are easier to live with, if they get handled a lot. Ask your wife to climb aboard your new yacht and sit on on uncoated aluminum seats and see how long this works.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    So why add the "throw away" comment if you can't be bothered arguing the point?! Clearly doing so suggests you are..

    Aluminium has excellent corrosion proprieties namely because of its protective oxide layer. Thus the key to the corrosion resistance of aluminium is protection of this oxide layer. The attack on this oxide layer is via a local change in the environmental pH. Thus whether such changes in the local pH level from a neutral pH to an acidic one or an alkaline one is critical. The mechanism is all that is different, that being a crevice, pitting, exfoliation, poultice etc, yet the end result is the same, the dissolution of the oxide layer - corrosion. Each mechanism is well understood and well known. One does not call a structural member of a hull that runs transversely a stringer, merely because it is also a 'bit of structure'...it has its correct definition that being a 'frame'. Similarly with corrosion, the mechanisms all have their own definitions but collectively called 'corrosion'.

    Poultice corrosion requires a wet wick-like materiel/substance to initiate it, as such one would not use this term for exfoliation corrosion simply because it has the same result! Crevice corrosion is almost identical in nature, but crevice corrosion does not require a hygroscopic material to be in direct contact.
     
  5. Barra
    Joined: Feb 2014
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    Barra Junior Member

    As a side note to the OP .

    I have also found that contrary to what Captain "Keyboard" may claim, ones sails don't end up any dirtier from the mast section being unpainted.

    The main sources of dirt on white sails are from rubbing on the shrouds and the worst by far is mould between layers of laminate sails. New laminate sails can look NASTY real quick after a couple of seasons in the tropics.

    Also instead of buying sail slide/track lubricant just rub a bar of dry soap on the track as high up as you can reach before hoisting (a layer seems to build up overtime even during the wet season. The bar will last years and it works a treat and keeps yucky aerosol stuff off you and your sails.
     
  6. mydauphin
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    mydauphin Senior Member

    I have one wonder about AL... Zinc-chromate... lots of it.
     

  7. jake101
    Joined: May 2016
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    Location: australia

    jake101 Junior Member

    We had a painted aluminium mast on a 43 foot Roberts. Covered in corrosion blisters.

    Sand blasted it and coated it with penatrol. 10 years later and no corrosion.

    Very happy
     
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