UV protection from paint

Discussion in 'Finishing Techniques' started by BBerson, Apr 27, 2008.

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  1. Apr 27, 2008 #1

    BBerson

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    Does anybody have actual scientific test data about how many coats of paint are needed to protect composite aircraft from UV damage?

    I was thinking UV is more energetic than visible light, so just thinking about opaque coverage for visible light might be irrelevant. UV is between X-rays and visible light for example. On the other hand a thin clear coat of sunblock rubbed on my skin protects from UV. Can anyone explain this?

    I want to know the minimum thickness of white paint to protect epoxy from UV damage without using any primer.
    BB
     
  2. Apr 27, 2008 #2

    Midniteoyl

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    Well... not really..

    SystemThree (a marine urethane paint) recommends at least 3 light coats for a total of 7.5 mills (0.0075") ( 3x 2.5 mils (0.0025") dry film thickness for maximum protection.
     
    Last edited: Apr 27, 2008
  3. Apr 27, 2008 #3

    Topaz

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    There are also dedicated UV barrier paints designed specifically for composites. They contain carbon black (yes, they're black or dark gray), so they absorb UV completely. The white thermal control and finishing coat(s) go over that.
     
  4. Apr 27, 2008 #4

    Midniteoyl

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    However, there is reason to believe 'black' is not the best choice as you want to 'reflect' the UV and not absorb. Absorbing really means 'changing' and this means at the molecular level. This is the main reason for 'dulling' and 'chalking' of paints and gelcoats.

    A 'good' system would use the black, absorbing, layer as a primer with a highly reflective top coat (2 step)

    Most 'better' quality systems in this class have a lighter color aborbing primer that has some reflective properties, a reflective 'sealer' coat, and a highly reflective top coat (3 step)
     
  5. Apr 27, 2008 #5

    Midniteoyl

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    http://www.ultralightnews.com/features/pdf/54NewsletterMarch2003.pdf (page 6)




    Also found this:

    http://www.hytechsales.com/insulating_paint_additives.html

    http://www.koolcoat.com/ ..hmm, I wonder.. :)
     
    Last edited: Apr 27, 2008
  6. Apr 27, 2008 #6

    BBerson

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    But how do you know that carbon black blocks UV?
    The properties of UV must be different from the visible light that we are able to see.
    The titanium dioxide in white paint might be good enough without any primer.
    I don't trust the paint dealers. They need to sell multiple coat "systems" to make more profit. This just adds weight and wasted time if not needed.
     
    Last edited: Apr 27, 2008
  7. Apr 27, 2008 #7

    Midniteoyl

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    Just about anything will block UV.. the question is do you want to block it by absorbing or reflecting? I like reflecting.

    You really should use a primer/surfacer with a sealer over that before painting. It makes the difference in a 'good' job that lasts 10 years, or a 'great' job that lasts 25 or more..
     
  8. Apr 27, 2008 #8

    BBerson

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    Midniteoyl
    I just read the article about UV that you provided. Thanks, that is the sort of info I was looking for.

    I prefer to use the same paint for base coat and top coat. In my experience, primer surfacers are filled with talc to make sanding easier but this makes for poor bond. Have you noticed all the older GM pickup trucks on the road with paint that blew off and the grey primer is visible?
    Thanks for the article link.
    Now I just need to know how many coats are enough to block the UV.
    BB
     
  9. Apr 27, 2008 #9

    Topaz

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    Well, I can't answer the "why"'s, but the type of system I described has been standard on hand layup composites since Rutan's Eze's came out. As far as I know, that's the way it's done: Primer coat, UV block coat (dark grey, based upon carbon black), then white 'color' coats on top for appearance and thermal control.

    Check with any Eze builder, or Cozy, etc. AFAIK, that or some similar variant is what everyone is using.
     
  10. Apr 27, 2008 #10

    Midniteoyl

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    Exactly... the first primer 'coats' are for sanding, smoothing, feathering, etc... you then hit it with a primer 'sealer' that dries hard to 'seal' the sanding primer so paint will actually stick. If you use primer/sealer first, it makes for really tough sanding and will eat through your supply of sandpaper. Remember, most of the first coats of primer will be sanded off as you smooth and fill the low spots, and primer is lighter than paint, so there's really no net weight gain.

    Understood.. it does work and is still the most widley used way of doing it. However, current thinking is slowly going to the use of barrier paints who's make up reflect rather than absorb for reduced heat build up and a longer lasting finish. The other affect of this is a reduction in the number of top coats required to cover since this coat can now be colored.

    Tires use carbon black as UV protection, and we all know what happens to them if left in the sun :)
     
  11. Apr 28, 2008 #11

    Topaz

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    Huh. Interesting, and thanks for the info. I'll be asking more about this when the time comes to paint my own airplane!
     
  12. Apr 28, 2008 #12

    PTAirco

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    What about the simple method of adding aluminium powder to the paint? It used to be the most common method for fabric and I see no reason why it shouldn't work just as well on composites. It could be used like a gelcoat in molds, or sprayed on as the final layer on a finished surface.
    If you mix the powder into pure resin in very high concentrations you end up with a coating that can even be polished and at first glance appears to be an aluminium surface.
     
  13. Apr 28, 2008 #13

    Topaz

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    Well, someone with more current information may correct me on this, but I think it's a matter of heat. White stays the coolest, and anything darker will heat up more in the sun. With hand-layup methods, the direct-sun surface temps for non-white colors can easily exceed the point where the epoxies start to soften and creep. Especially out here in the west where we both live.

    So the mantra ends up as a paraphrase of the old Ford dictum on color: "You can have any color you want, as long as it's white."
     
  14. Apr 28, 2008 #14

    BBerson

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    I tried mixing in aluminum powder because that is what we did with fabric. I noticed the silver color was much hotter than the nearby white in direct sun.
    But I have not found anything in the books that advise use of aluminum powder to protect composite.

    I think the pigment in any color works as well as aluminum. It is more convenient to only use white for all coats. It takes less paint and repair is easier, so why add aluminum?
    At composite seminars, I ask about UV protection. So far, no answer.
    One guy in the audience said clear coats have additives for UV, otherwise the clear would degrade rapidly. Solid colors are better obviously and may have UV additives as well.
     
    Last edited: Apr 28, 2008
  15. Apr 28, 2008 #15

    Midniteoyl

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    Thought about aluminum too.. but as the 'barrier' coat, not top.. The problem then arises with were to place antenna on fiberglass planes. I mean, thats a big plus being able to place them internally. Carbon planes I guess it doesn't matter too much...

    But, as said above, aluminum transfers heat more readily, so it wouldn't be very good on composites as a top coat..
     
  16. Apr 28, 2008 #16

    Midniteoyl

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    As an aside.. what about adding ceramics to the paint? Supposedly they help stop heat transfer, little bit of sound deading, and reject UV. The texture of the paint is suppose to be 'velvety' or like suede with millions of tiny 'bumps' in the finish (if used in the top coat). Wouldnt these 'bumps' help keep the boundery layer attached to the wings kinda like 'shark skin'? Would be interesting to test...
     
  17. Apr 28, 2008 #17

    bmcj

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    I believe Stits has some 100% UV coats in colors and possibly in clear.

    By the way, if things haven't changed, I believe that all white paints still get their "white" from Titanium. I wonder if it is enough to give UV protection...seems like if one metal (aluminum) works, another (titanium) might too.

    Bruce :)
     
  18. Apr 28, 2008 #18

    PTAirco

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    I got the notion of the aluminum powder for composites when I saw the stuff for sale at a Marine West store, presumably for use with resin.
     
  19. Apr 28, 2008 #19

    bmcj

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    Though sometimes hard to find, aluminum powder has always been available for those that wish to mix their own silver coat.
     
  20. Apr 29, 2008 #20

    JMillar

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    The problem with Al powder in terms of availability, is that it's a common pyro ingredient. Many home made fireworks mixes, etc, use aluminum powder since it's a very good fuel. It can even reduce rust in a violent reaction (thermite process). So selling it invites liability.

    I deny whatever you're thinking about how I know that.
     

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