Gelcoat or PU coating on composite?

Discussion in 'Finishing Techniques' started by Birdman100, Jun 14, 2014.

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  1. Jun 14, 2014 #1

    Birdman100

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    It seem that Polyurethane paints and lacquers are becoming more frequent choice for composite surfaces instead of gelcoats.

    Am I right about that and what do you think is the reason? Surface quality? Weather (UV) resistance? Weight? Processing? Cost? Other?

    As far as I know there was a lot of Polyester gelcoats use, especially in gliders, but why there is absence of epoxy gelcoat applications?

    I know that gelcoats are heavy (typical about 500g/m2), but they can be still applied as a thinner coatings...
     
  2. Jun 14, 2014 #2

    JamesG

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    Gelcoating began as a technique in marine (boat) composites, where it originated. Also with other fiberglass production items like car bodies, etc. When fiberglass composite aircraft started to be made, the fabrication/finish techniques were incorporated in it because... that was the way it was done (but not in aerospace). It simplifies the production process, but its not ideal for aircraft use. Which is why it's become less and less used.
     
  3. Jun 14, 2014 #3

    Birdman100

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    This is historical reason, ok. But there must be some technical aspects as well - PU vs UP gelcoat?

    Fiberglass technology were incorporated in aircraft industry more than 40 years ago... And some sailplane manufactures still use gelcoating (?)

    If you were to build CFRP part (wing) what would you proposed as a surface finish layer?

    1) raw carbon laminate,
    2) sanding,
    3) primer,
    4) 2 coats of PU...

    ?
     
  4. Jun 14, 2014 #4

    Topaz

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  5. Jun 14, 2014 #5

    JamesG

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    Because it was cheap both in material (you can hide a lot of flaws and shortcuts behind a layer of goo) and time (labor). That is the only reason. Where that isn't a concern or for high performance (aerospace and high-end racing) gelcoat is never used.
     
  6. Jun 14, 2014 #6

    Birdman100

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    Thanks Topaz, really good info!

    This is more on filling and sanding, so it refers more to a repairing work. What about new parts going right from the mold? Guess, we should avoid any filler work if molds are properly designed and with nice polished surfaces...

    What do you think about PPG or Standox car paint systems? These are polyacrylate I think not the PU.

    Thanks. Finishing of the high performance (new) parts is what interests me. Goal is low-weight (200g/m2 primer+paint is that too optimistic?), hard, UV resistant, high gloss, long-lasting surface. Guess, thats the way latest competition sailplanes are finished. Any more info is welcomed:)
     
  7. Jun 15, 2014 #7

    TFF

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    Cirrus planes are painted with PPG base clear.
     
  8. Jun 15, 2014 #8

    autoreply

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    Indeed. Gelcoat is one of the reasons many LSA's/microlights are terribly overweight (350 -450 lbs payload). Was slightly amusing to have lots of guys from other factories gaze over our carbon outside (waiting for a paint job). Looking into many of those aircraft (wheel wells, inspection holes) and you see what a mess the layup often is.

    This article is a bit biased, but gives a reasonable overview:
    DG Flugzeugbau: Polyurethane Paint - The Better Alternative

    By far the lightest is a transparant PU coat.
     
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  9. Jun 15, 2014 #9

    Birdman100

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    Im safe with the PPG then. BTW, their finish is amazing.

    Thanks, I read this a couple of years ago. Good to recall. Many useful info at their site anyway.


    Is that sufficient UV protection for laminate? How would you deal with the black carbon surface underneath and overheating by the sun radiation?
     
  10. Jun 15, 2014 #10

    dino

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    [FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]I read in the Sailing Anarchy Forum that some builders were applying paints as a first coat in the laminating process instead of gel coat. PPG 700 and 1000 were mentioned which are industrial epoxy/siloxane hybrid paints and applied directly on the mold release surface in 4 coats before laminating.[/FONT]

    [FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]Reportedly PU paints have been used as initial coats on molds and PU gelcoats (thicker) as well [/FONT]

    [FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]Dino[/FONT]
     
  11. Jun 15, 2014 #11

    autoreply

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    Birdman, this discussion ran almost parallel to your question:
     
  12. Jun 15, 2014 #12

    Aircar

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    Gelcoats are harder than PU paint and are impermeable to water (PU is NOT) --the post repair refinishing of gelcoat can be done with no visible evidence but polishing out of polyurethans is not as easy or even possible with some types (you may need to repaint an entire component -cutting the gloss skin loses the finish , old DUCO and similar lacquers could be polished to an equal finish but modern 'gloss off the gun' systems can be troublesome . We encountered a problem (in Australia) about thirty years ago with micro cracking (crazing) on some German glass glider gel coats due to UV light and higher temperatures causing shrinkage --stripping of wing upper surfaces and re finishing was imposed on many gliders but was of dubious value. Polshing of wings with silicone waxes was a real problem in getting adhesion of a replacement paint -- the worry was that the cracking in gel coats would progress into the UD glass skins and at the least provide a pathway for water to enter the structure . Two different types of gelcoat were involved then (now superceded) Vorgelat and Schwabblack and had different lives and crack propensity . Gelcoat also showed damage that would not be apparent under a more flexible paint coating .
     
  13. Jun 15, 2014 #13

    BBerson

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    Polyurethane is superior but costs more.
    I spent months sanding off gel coat and repainted with white polyurethane (Dupont Imron).

    The owner of Stits Aircraft Coatings told me that statements about clear coats UV resistance are pure fiction (he used other words).

    I am still waiting for independent, credible UV lab test results, not opinions from sales people.
     
  14. Jun 15, 2014 #14

    autoreply

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    Much of the issues Aircar speak about are related to post-curing of the structure itself. The famous spar bumps (spar cap replaces the sandwich in the skin where the spar runs) will change shape a bit until they're totally cured (months or even years later.
    Virtually all experienced manufacturers now have a light (25g/m2) glass outer layer, woven, to stop any cracking. With infusion that one becomes invisible.
    Old gelcoats (some of them) are far superior to what we have now (environmental BS). Virtually all Glasfl├╝gels still have their original gelcoat, 50+ years after they are made, even those who fly in wave (-30C) from hot airports (40C). Some younger sailplanes sometimes need a repaint after as little as 15-20 years.

    The guy I spoke to was an engineer and I know one of their test (lab) guys too (they're within walking distance from my house). I wouldn't worry about that too much if they use it in that way on (FAR/CS25) certified aircraft. Pigment there is under the PU coat and can't stand UV either)
     
  15. Jun 15, 2014 #15

    BBerson

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    This comment isn't clear (hehe).
    Are you saying that one coat of clear polyurethane provides enough UV protection for certified aircraft?
     
  16. Jun 15, 2014 #16

    autoreply

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    Yep, according to them, such a transparant coat of 60 micron is enough, under the condition that the original surface is very smooth already (2000 grit).

    Switching over to speculation now, but it seems they mostly cater to big aviation (FAR/CS25). That might be an obstacle to homebuilts.
     
  17. Jun 15, 2014 #17

    BBerson

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    Micron = .000039"
    so 60 x .000039" = .002" coating thickness.

    I think that could be one heavy coat. But the dry thickness is different for every product.
     
  18. Jun 15, 2014 #18

    Birdman100

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    Heavy??

    70 g/m2 - ten times less compared to gelcoat.
     
  19. Jun 15, 2014 #19

    BBerson

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    Heavy, as in as thick as spraying the paint is possible without runs. Polyurethane process is much lighter.
    Molded Gelcoat is thick and must be sprayed ten times thicker (.020") as I understand, to avoid wrinkling.
     
  20. Oct 8, 2014 #20

    tunnels

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    Yes gel coat is heavier than most paints and no it does no have to be thick at all . It is possible to spray a thin coat of gel coat but have to catalyse I little higher than normal also warm your mould to promote faster gel and much better cure!! Cure is the key to gel coat success and finish quality !
    The wrinkling you refer to is caused usually by one or both of just two things first is the gel is under cured and the styrene attacks and makes the gell wrinkle!!,
    second and this can happen in patch's, is slow gelling resin ! so need a gel time of close to 15 to 20 minutes no longer!!
     

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