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Painting aluminum masts


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  #1 paul_stewart

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Posted 18 September 2008 - 08:54 AM


The problem with aluminum is that it is difficult to fully remove the oxide surface with sandpaper and not embed some of it in the aluminum itself, where it can still interfere w/ adhesion of some coatings.  Chromate conversion coatings (the "chem-film" I mentioned) are the standard, where, after sanding, machining, etc., you treat w/ the chromate solution to convert a thin film of aluminum to a very adherent phosphate surface that has excellent adhesion qualities for organic coatings (paint).  The application of an epoxy or paint primer to a sanded surface is reliant upon only the mechanical "grip" of the cured coating to the metal and is vulnerable to details of microdebris like sanding residue, humidity, trace solvents, etc.  Sometimes it may get a good grip, sometimes not.  I'd agree that epoxy has a better track record at getting that initial grip than paint alone.

"Self etching" primers like Vinyl-Lux are a conversion coating, making a new chemical bond w/ the aluminum and presenting a "friendly" surface for further coatings.  It's intended for only a single thin coat, because the coating reacts only at the interface between the solution and the metal.  Careful, the stuff contains phosphoric acid and metals, so don't use bare hands and dispose of any waste properly.  Look at the instructions for coatings on Interlux's site.




If the primer is flaking away with the topcoat, then my guess is the priming step was the problem.  I would probably have used an aluminum etching primer wash, like Interlux's zinc chromate Vinyl-Lux before applying the System 3 primer.  Getting good adhesion to aluminum can be a trick.  Chromate primers require careful use for safety/environmental reasons, but work.


Brad,

Thanks for your helpful insights. 

Areas on my masts that rub other parts (e.g. sprit booms) have peeled.  I still have some System3 Yacht Primer and System3 Topcoat Paint left over from painting our aluminum masts back in February 2007.  Would be best to "get it right" before spending a lot of time "getting it wrong"  again. 

The System3 Yacht Primer and the 2-part System3 Topcoat Paint have peeled off together,  so that means the primer lost its grip on the aluminum. 

The area could be prepared (sanded, etc.) and a "self etching" primer, say Interlux's zinc chromate Vinyl-Lux primer,  could be applied as you suggested. 

My question is, "what next?":  should the System3 Yacht Primer be applied over the Vinyl-Lux?  Or, alternatively should System3 Topcoat Paint go directly on top of the Vinyl-Lux.  It may or may not be feasible to get an answer to this question from the experts at System3 and Interlux. 

You suggested putting System3 Yacht Primer on top of the Vinyl-Lux primer.  Any particular reason for that suggestion?

An alternative would be to strip the aluminum mast of all coatings and start over with Vinyl-Lux followed by whatever Interlux recommends as a top coat paint.

Any further thoughts or suggestions?

--Paul


  #2 Guest_drake_*

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Posted 18 September 2008 - 02:07 PM

I don't mean to hijack the thread and hope you don't mind a stupid question for my first post on this forum, but: why paint the aluminum mast at all?  I've never owned an aluminum mast that was painted and can't recall that many of the boats I've been on or around have them painted.

I just don't like to go through a bunch of work and deal with peeling paint (and preventing it) if there isn't a good reason to do it.  Thanks for your thoughts.

  #3 Charlie Jones

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Posted 18 September 2008 - 02:13 PM

Virtually all of the production masts you see are anodized. That is a significant protection. Without the anodizing the aluminum on the surface will oxidize and leave a white powdery film on every thing. So we paint. Oxidation on the aluminum is basically the same as rust on steel. If you wash or wipe it off, it comes back, and eventually the aluminum goes away. Might take years, but go away it will.

I've had a LOT of success with Zinc Chromate as a primer by the way. Then either enamel or 2 part poly goes over and holds up quite well.. I've done 4 or 5 masts like that and none have given trouble.

  #4 BradW

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Posted 18 September 2008 - 02:46 PM

The acid etch primer (there are others...you can try auto paint shop suppliers) provides a tie coat for the primer. Vinyl-Lux is Interlux's version of a "wash" primer.  SherwinWilliams and others make them too.  It is very thin and is there only to make the aluminum surface "convert" to a stable adherent film that organics (epoxy) can bond to reliably. This is in place of a "conversion" treatment like Alodine. The epoxy primer is there to provide longterm corrosion resistance, build in some cases, and a friendly surface for the polyurethane. BTW, Interlux recommends blasting rather than sanding, and vacuuming the dust off rather than wiping with rags/solvent.  I think the System 3 primer is an epoxy primer like other high quality ones.  If you look at the literature on the site, I think they also recommend an acid etch before the primer, probably for similar reasons to Interlux. 



  #5 paul_stewart

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Posted 19 September 2008 - 06:58 AM

Brad, Charlie and Drake,

     Thanks for the very helpful input! 

     Painting our aluminum-tube masts seems to be the best option for protecting them from cumulative corrosion; anodizing would not be feasible for almost all of us builders of home-built boats.
       As you explained, Charlie, unprotected aluminum will evenually "go away" because weathering both creates a protective layer of aluminum oxide (called "alumina")  and takes it away:
       "In dry air, the natural film will consist only of the barrier layer and will form rapidly to the limiting thickness.  On the other hand, if the destructive forces are too strong, the oxide will be hydrated faster than it is formed, and little barrier will remain."   
         ---from http://www.key-to-me...m/Article14.htm

      Aluminum is strange and wonderful stuff.  From wikipedia....   
     "Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of alumina quickly forms on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising.   ....
       Anodized aluminium surfaces are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating.  ....
      Anodization changes the microscopic texture of the surface and can change the crystal structure of the metal near the surface. Coatings are often porous, even when thick, so a sealing process is often needed to achieve corrosion resistance. The process is called 'anodizing' because the part to be treated forms the anode electrode of an electrical circuit.  ....
      Corundum is the most common naturally-occurring crystalline form of aluminium oxide.  The gem-quality forms of corundum are rubies and sapphires.
      Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity."

      --- http://en.wikipedia....Aluminium_oxide
      --- http://en.wikipedia....odized_aluminum
      --- http://en.wikipedia.org/wiki/Aluminium


      By the way, alumina is an electrical insulator so the surfaces of an anodized mast must not be a very good conductor of lightning.  What are the implications of that?

Also from wikipedia,  the carabiners pictured below are anodized and dyed.

Attached Files



  #6 wardm

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Posted 19 September 2008 - 02:52 PM

Another option that is much less work than painting, less costly, and lasts very well is to use Tectlyl 151 it is a Valvoline product that useally comes in a spray pack. It is not as hard wearing as Anodise, but for the home build it is a cheap, quick and effective option for masts, booms, etc.

I usually clean the aluminium fist with an Acid wash, then coat with Tectyl 151

Tectyl is used by mast builders and tinny (aluminium dinghy) makers in Australia.

Can also be applied with a rag. Do multiple coats to get a good thickness, it dries clear and retains the shiny look from the acid wash.

Further down the track if wanting to re apply it is recoatable into itself.

  #7 paul_stewart

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Posted 19 September 2008 - 03:39 PM

Ward,

The preparatory "acid wash"  you mentioned means ......... what?    A special acid wash?

The Valvoline Tectyl 151 seems to be a little hard to find around here; I found a website that will provide a quote for a similar Tectyl 155   --how many truck-loads would we like?


To summarize: After cleaning the aluminum the chemical-grip methods discussed have two steps...
     --anodize  and then  dye and/or sealant
     --Brad's chromate conversion coatings ("chem-film")  and then  some kind of paint   
     --Brad's self-etching zinc chromate primer film Vinyl-Lux   and then   Yacht Primer+Topcoat Paint
     --Charlie's zinc chromate primer  (Vinyl-Lux?)  and then   some paint
     --Ward's acid wash   and then  Valvoline Tectyl coatings
The mechanical grip methods...
     --sand + epoxy coat      and then  Yacht Primer + Topcoat Paint
     --sand + cross fingers    and then   Yacht Primer + Topcoat Paint


--Paul

  #8 Ken_Potts

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Posted 19 September 2008 - 08:59 PM

   Paul,
   Don't forget that 6061-T6 Tubing is anodized to start with.  If you strip the anodizing to prep for paint are you really getting more protection?  If paint is better than anodizing how are you going to paint the inside of the tubing?
   The outside of the tubing gets more abrasion overall so the anodizing will wear off most of the outside of the masts before it wears off most of the inside of the masts.  Adding extra protection to the outside will still extend the life of the masts though, right?.  On the other hand, there are plugs at the base of the masts that fit inside.  Those plugs are abrading the inside of the masts at the very base where they fit into the steps.  You don't need to degrade the entire length of the mast to have a failure.
   I'm not advocating painting or not painting, I'm just wondering if painting has a positive effect on the durability of the masts.
   I'm speaking from a purely theoretical standpoint here and I'm looking forward to learning from the responses of people who have been at this longer than I have and have seen the real effects. (somebody shoot me down, I want to learn something)  ;)
   Oh yeah - Let's go sailing!  ;D

  #9 paul_stewart

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Posted 19 September 2008 - 11:19 PM

Ken,
     Good points.  I too would like to know more....    As most of us on the B&B forum have built or will be building masts from aluminum tube stock,  gathering some facts here in one thread on the forum seems worthwhile.
     Not all aluminum tube stock is anodized; or perhaps it is? 
     The masts on our CS20 are made from tubes with 0.090" walls.  Came from Graham who found a source for it in Canada and was glad to stock some of it.  I don't know off-hand if the alloy = 6061-T6.   
     As I recall (all the way back to February'08), we painted our masts (white) primarily because we thought they would look better that way.  So whether painting adds longevity to the masts is/was not an important consideration in our case.   I like the white masts.   We might have avoided some peeling if we had known about the advantages of starting with a zinc chromate primer.
    I would guess that an anodized surface is "better" than a Vinyl-Lux+Primer+Painted surface,  but the process of joining the three segments of the mast with epoxy required some sanding which would have removed the anodizing in some spots  --if the tubes actually were anodized to start with.  Also, the original tube stock was not perfectly clean when we obtained it and probably had a few invisible scratches, etc.  So there can be several reasons for deciding to dress up the masts with some sort of coating(s)  --clear or colorful.
 
--Paul



  #10 Charlie Jones

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Posted 20 September 2008 - 05:39 AM

Just walked out to the shop and took a look at a few chunks of 6061-T61 left from a couple of masts. Sorry but there's no way those were ever anodized. They came from Online Metals by the way, an excellent supplier.

They are sitting in a corner right next to an offcut from a furler extrusion that IS anodized, so the difference is obvious.

On the question of the inside. Yes, the inside will oxidize. But then there will not be anything to wipe that oxidation away, so the layer will stay on the surface and protect the metal, which is it's purpose after all.l

And the mast plugs at the bottom are epoxied in place- which definitely seals the metal. So no wear occurs there either.

Now where you CAN get wear is in masts that are built in sections with the intent of separating the sections each time you sail. THERE you have exposed aluminum with no way to protect it- the act of sliding the sections together will abrade the surface. But that happens on ANY mast that comes apart made of aluminum, even anodized sections. So the answer there is to just keep it clean, perhaps keep some light lubrication on the surface and live with it.



  #11 Ken_Potts

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Posted 21 September 2008 - 07:17 PM

  Paul,
  I guess it's not all anodized.  The stuff I used was as far as I know (I was told it was, it had printing on it saying it was, etc).  It hasn't turned my luff-sleeves grey yet.  You've got me thinking that an advantage to painting the outside is that it will be obvious when the paint needs to be re-done, but the only way I'll know that it's time to re-coat my masts is when the sleeve-luffs turn grey.

  Charlie,
  I epoxied the plugs at the bases of my masts in and then one of them eventually fell out.  I think the wood moved a bit too much for the epoxy joint.  Now the plugs are screwed in place so they work against the inside surface of the masts, continuing to wear the anodizing off even as more builds up. That's why I mentioned the plugs. Eventually they will rub their way through the tubing but I don't know if that will be next year or 20 years from now.  I'm probably going to have a co-worker turn some aluminum plugs for the masts but then I'll still have aluminum wearing on aluminum.

In the meantime I've been out on the lake two days in a row.  Tomorrow's maintenence day. :)



  #12 Charlie Jones

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Posted 21 September 2008 - 07:35 PM

Well, it's perfectly obvious that you must do some serious research-

Sail the bejeezus outta the boat and see just how long it takes to wear it through ;D ;D ;D

  #13 Ken_Potts

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Posted 21 September 2008 - 07:43 PM

  You got it right there Charlie!  ;D
  Once I find the limits I'll rebuild the broken thwart, saw three inches off the base of the mast and start all over again...
  And I'll post the pics of the rescue too!  ;D




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