Can Aluminum be painted?

Discussion in 'Metal Boat Building' started by jdworld, Mar 4, 2010.

  1. jdworld
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    jdworld Junior Member

    Can aluminum be painted? I know it can be anodized, but can it be painted - ie like a car? (For instance, on a hull to be used only in freshwater)
     
  2. Landlubber
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    Landlubber Senior Member

    Yes it can (get it can....see Coca Cola for instance).

    The trick is to get a really good primer onto it, I prefer the alloy to be anodised fiirst if this is at all possible.

    We used to use zinc cromate for the primer, but it appears to have gone with the do gooders up the creek.

    Altex Devoe make superb alloy paints and primers, so I would suggest that you examine thier website.

    The epoxy based primers make great substrates for two pack linear polyuretnane finishes if you are looking for maximum gloss and long life.

    I painted a dive boat and it was sold ten years later, the new owner asked if we painted it to sell it, so you can do a good job if you wish to.
     
  3. messabout
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    messabout Senior Member

    Various primers, available at automotive paint supply stores, sometimes have a phosphoric acid component that is very effective. My company has been painting aluminum measuring instruments for 25 years. No problem with the right primers.
     
  4. Joe Petrich
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    Joe Petrich Designer

    Acid etch (Alodyne is one), prime, then topcoat.
     
  5. Ike
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    Ike Senior Member

    To paint aluminum is tedious and must be done in a painstakingly careful way. Alum has a natural oxide on it that protects it from corrosion. It is transparent, you can't see it but it is there. For paint to stick you must remove the oxide by etching and them paint with a good aluminum primer and then with a good paint that will last. Other wise the paint will not stick to the aluminum and in no time at all it will look like crap.

    Here is something I wrote on this a few years back:
    http://newboatbuilders.com/docs/aluminum.pdf
     
  6. jdworld
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    jdworld Junior Member

    "Alum has a natural oxide on it that protects it from corrosion. It is transparent, you can't see it but it is there. For paint to stick you must remove the oxide by etching and them paint with a good aluminum primer and then with a good paint that will last. "

    All good to know.....and I'm encouraged that it IS in fact possible. But one question, if you etch off the oxide which protects from corrosion just to add back the semi-protection of paint......aren't you left with something that will corrode under the paint and in places where the paint gets scratched off ie like steel? Or does the oxide re-form in those places and protect the aluminum?
     
  7. Ike
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    Ike Senior Member

    As they say AHA! The point of the oxide is to exclude oxygen. This prevents oxidation (corrosion) The point of the paint is to make it look pretty, but to keep it looking pretty you also have to exclude the oxygen. Otherwise you are right, it corrodes under the paint. That is why you see on painted aluminum those flaky bubbly areas, that when you scrape them away reveal a white powder, aluminum oxide. If you nick or ding your paint you have to restore it right away.

    So if you don't do it right, in a short while you have a crappy looking aluminum boat. That's why most aluminum boats are bare.
     
  8. CDK
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    CDK retired engineer

    Aluminium and Magnesium react with oxygen immediately, so removing the oxide is possible only in an inert environment. Because of that property alu con only be welded under a liquid salt or protective gas.

    Etching creates a rough surface for a stronger mechanical bond and removes contamination, after rinsing the oxide layer is back in place.

    The white stuff under a blister is not aluminium oxide but a salt complex mixture containing Na, Al, O and Cl. It is only formed under a skin that prohibits formation of Al2O3 through lack of oxygen.
     
  9. jdworld
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    jdworld Junior Member

    ok, so in plain english, if you have a nice shiny painted alu hull, and you scrape a rock while beaching it, it that scrape going to "rust out" at the same pace as an identical steel hull in identical conditions?
     
  10. jonr
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    jonr Senior Member

    No, it will just oxidize and thereby protect itself from further corrosion.
     
  11. BMcF
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    BMcF Senior Member

    We have had very good results with an epoxy strontium chromate primer for aluminum from Pro Line (owned by Sherwin Williams if I'm not mistaken). But..to get the results we do, we acid etch the entire area to be primed and prime as soon as possible after that. So its a royal PITA for larger hulls to deal with th acid etching part of the problem with proper respect for the environment.

    Done correctly..the Pro Line stuff holds up very well. Some blistering of the topcats will be seen only locally around stainless hardware or fasteners and that will appear within 4 to 5 years and can be spot-repaired if necessary.
     
  12. jonr
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    jonr Senior Member

    You can also anodize (smaller pieces) and then dye and clear coat the anodized layer. Can create a very nice non-paint look.
     
  13. kmorin
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    kmorin Senior Member

    Painting Aluminum Steps

    jdworld,
    one step in the process that hasn't been stressed- to its level of importance- is the removal of aluminum oxide and substitution of another metallic oxide: CONVERSION. This is done to hold the primer on one side while bonding to the aluminum on the other.

    Chromium oxide is a good substitution for aluminum oxide and aluminum is 'converted' to this oxide by washing/rinsing/immersing the alloy in an acid that will remove the aluminum oxide.

    The acid -hydroflouric acid, most commonly- is not very user friendly and is toxic. But it cleans aluminum of its naturally occurring oxide very well. To remove the acid you rinse the metal with flowing fresh water and keep the water on the metal to seal it from the air - if air gets to the aluminum it will oxidize again and you'd have to repeat the acid etch.

    Then, while the metal is wet - flood/spray/dip the metal in Allodyne or another brand name of chromate solution to provide the 'pure'/unoxidized aluminum molecules with a chrome based molecule that will also be the new 'oxidizing' agent. This will form a chrome and oxygen [and] aluminum layer that will hold paint well and keep the underlying aluminum from contact with corrosives.

    This method is sort of a poor-boy anodizing and results in a surface that will hold primer much better than aluminum oxide - its called conversion because one metal oxide is converted to another metal oxide.

    If any area of the parent metal is not converted to the chrome oxide it will show a color change from the surrounding areas which are most often golden or greenish - the unconverted areas are blue.

    This coating, once dry can be painted with good results. Some primers [claim to??] perform this conversion without the acid etch/rinse/chromate flooding steps. I've used this critical step in painting aluminum boats since 1978 with good results, not that we've painted every boat built, but this was the method used to paint all that did get coated.

    A large boat can be painted by spraying acid, leaving it to foam and lift the oxide, then rinsing continuously with a water hose keeping the surface wet and then spraying Allodyne on the wet surface. If this is done in 3-4' wide strips up the hull, keel to cabin, with a slight overlap of acid used to carefully lift the dried edge of the chrome oxide area adjacent- then the entire boat can be done in manageable areas of work. Keeping the entire boat coated or wet in the rinse cycle isn't realistic without immersion tanks.

    This method won't work in the slightest breeze- if acid overspray spots the converted areas -that area has to be redone or the paint job will fail at those spots.

    Painting aluminum boats is a pain in the stern. It requires full rain suits- boots and all- with double gloves to handle the acid, Allodyne is toxic and can't be released-let alone inhaled-so you should work on a plastic pond liner under the boat and EVERYone should have a half face rubber sealing air purifying respirator not some silly suicide mask made of paper. If you can't muster the Personal Protective Equipment it would be better to skip the whole process rather than take the risks to all involved from chemicals required to do the work correctly.

    I also require everyone to have splash goggles as well as a face shield since if acid gets on the face with ordinary safety glasses it can run down the forehead behind the glasses and get into the eyes- this stuff [aluminum cleaning acid] is not casual and should be treated with respect- bordering on fear.

    To clarify one of Ike's points about aluminum tanks

    If the tank is in a free air circulating bilge area, without bilge water or deck leaks or even condensation -and the tops are sloped enough to drain -then painting is optional. But if the tank is bedded in foam [poor practice but regularly done], under a plank deck that may leak, or can be wetted by the bilge water - then painting to the primer stage is very good practice.

    The question is; will the surfaces of the tanks be allowed to dry out and can the self-healing aluminum oxide stay intact? If so, then paint is optional and not required. If not, if there will be any lack of air, constant wetted surfaces or any bilge water slop then the paint will help the aluminum to maintain without as much possibility of corrosion due to some agent attacking the aluminum oxide.

    cheers,
    Kevin Morin
     
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  14. BMcF
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    BMcF Senior Member

    Very good summary and advice!. I did not elaborate much in my post..but it is hydroflouric acid to which I was referring to when I mentioned the acid wash part. Nasty stuff indeed.

    Chromate conversion (alodyne, brand name, but now in common useage for the 5541 process regardless) is a superb way to go..probably the best overall IMO. We do use that for all 'small parts' but it's pretty expensive for an entire hull ...about $65/gallon from Dupont recently.

    That said, the ProLine strontium chromate does react and convert the aluminum IF the ambient air temp is right, humidity is low, and we manage to spray the primer immediately after the aluminum dries enough after the acid wash.

    We even use test panels (acid washed and painted at the same time) to see how well we did; strip the epoxy paint off with regular paint stripper about 3-5 days later...if the process 'worked' the aluminum will look exactly as alodyned aluminum does with a nice golden sheen, If we were tardy with the spray application, not agresseive enough with acid, etc etc...removing the epoxy reveals..plain ole bare aluminum.:rolleyes:

    We are getting ready to redo a 32' Marinette, entire bottom to the boot stripe. Yuk.;)
     

  15. Brent Swain
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    Brent Swain Member

    A friend, who builds aluminium boats, said his etch primer was softer than the epoxy over it, so it chipped easily where the etch primer was. It stuck far better where it had been lightly sandblasted. He said the only really reliable way to get paint to stick to aluminium was to sandblast it. Etching is a disaster on steel, as the acid keeps working under the paint, causing huge and ongoing corrosion problems . I've seen no exceptions in the last 38 years .
    In the tropics , if you don't paint aluminium white , it will get hot enough in the sun to fry eggs on, and will burn the soles off your feet. I had to paint the inside of my aluminium dinghy to stop it from burning my feet.
     
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