Building From Mold without Gelcoat

Discussion in 'Fiberglass and Composite Boat Building' started by weldandglass, Apr 10, 2017.

  1. fallguy
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    fallguy Senior Member

    I did read the ops use of the primer incorrectly. Thanks.

    Isn't mat typically not used with epoxy, too? I thought mat had a binder that breaks down in esters; not epoxies.

    His original post says he was using epoxy Groper, so I was just asking about post cure because System Three engineers have told me in the last 30 days that post curing is important. RX posted MOST epoxies don't have post cure beyond rt; but I was just told by someone it is vital.

    You really don't need to repost my lack of expertise; it is offensive. I'm here for help and clarity.

    I'm going to go back and edit that which might have been offensive to you because it was simply an error on my part.
     
  2. jorgepease
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    jorgepease Senior Member

    He's just trying to help.

    Infusion epoxies are usually the ones that benefit from post cure, laminating epoxies, I never heard they needed post cure till you mentioned it. Nobody on the stitch glue forums have ever done a post cure so if it's vital ...

    Post cure can be as simple as covering with a tarp and leaving it in the sun, otherwise make a tent and use a 100 lightbulbs or so ... actually it's supposed to be a regulated temp increase but that is pretty easy to achieve.
     
  3. groper
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    groper Senior Member

    I concur with what RX said about post cure. Most epoxies available here do not need a post cure... The ones that do need it are typically very long open time (+2hour pot life) infusion types which also have a very low viscosity. They remain brittle until they get a proper post cure. If system 3 is still using chemistry that requires a post cure on a standard hand laminating epoxy -I respectfully suggest you find another resin supplier...
     
  4. ondarvr
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    ondarvr Senior Member

    This has run a bit off topic, but useful information.

    Virtually every epoxy benefits from a being post cured, the longer the gel time the more important it is. Most data sheets list the physical properties for the epoxy after being post cured, don't expect to get close to them without a post cure. Some list them both ways so the user has a good idea of the difference.

    After saying that, epoxies tend to be strong enough that most people don't need to post cure them for typical uses, Companies that build high end products that need to meet a spec do post cure. Building a small skiff would not require a post cure with a typical laminating epoxy to make a very good and usable product.

    VE's and polyesters can benefit from being post cured, but not to the same extent.

    Using an in-mold primer is a personal choice in the production method. Sanding on a raw laminate opens up many small defects and air bubbles in the laminate, plus the glass fibers, these can be difficult to cover and hide when spraying a primer over them. Using an in-mold primer simplifies this step, there are no small defects to try and cover up, you can just sand and paint. The benefit is even greater when the mold surface is good, on a rough mold surface the primer may not help as much because a great deal of filling and fairing may need to be done. In this case the primer may be sanded off, or buried under filler.

    On a good mold surface any shrink or fiber print in the resin can typically be sanded out easily when an in-mold primer is used, post curing before sanding lets all the surface movement take place prior to sanding, so long term the surface profile is typically better.

    In reality it's not going to make much of difference in what resin or method is used to build a small one-off skiff, the end result will be close to the same and will depend more on the attention to detail of the builder than the exact resin or method.
     
  5. fallguy
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    fallguy Senior Member

    Okay, if the op doesn't mind my asking, what does 'in mold' primer mean? How is that done?
     
  6. jorgepease
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    jorgepease Senior Member

    x2 ...It must be a gelcoat type thing
     
  7. groper
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    groper Senior Member

    See above
     
  8. ondarvr
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    ondarvr Senior Member

    In-mold = sprayed in the mold and laminated on, just like gel coat.

    When discussing coatings, especially with epoxy builders, you need to say whether it's an in-mold or post coated product, the primers discussed can be used either way.
     
  9. jorgepease
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    jorgepease Senior Member

    How much time do you have to load the mold before it sets up? I doubt I could load a mold and get it ready for infusion in less than 3 days.
     
  10. ondarvr
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    ondarvr Senior Member

    You need to let the primer cure to tack free (they're designed to) before using with epoxy, after that you have about as much time as you need.

    For use with polyester or VE the primer shouldn't dry to tack free.
     
  11. fallguy
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    fallguy Senior Member

    So, the epoxy is providing the bond strength and not the primer. Very counter-intuitive (essentially gluing onto dried paint) and that is why I asked how. Very interesting stuff-sorry if I seemed to chuck a hand grenade. Just had visions of cooking paint early..
     
  12. rxcomposite
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    rxcomposite Senior Member

    I would not recommend that to the boat building community. First, sanding to the glass fibers exposes the fibers to the water and provides a pathway for the water molecule to pass thru aiding osmosis. Any damage to the fiber is a no no. The key is to provide a reliable water barrier.

    Second, anything that has "small defects" is bad workmanship and is a headache both for the designer and meeting Class regulations as well. "Print through" is acceptable as long as there is only 1 to 2 % void in the laminate. It is generally accepted that in print tru, the tip is still true to the mold surface and only the valley caused by resin shrink needs to be filled up.

    Voids, bubbles, pinholes should be avoided at all cost. It is only in commercial applications where it is not subjected to continous water contact where you can get away with it. Still bad workmanship though.
     
  13. fallguy
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    fallguy Senior Member

    So, rx, are you suggesting a resin fill is better?
     
  14. rxcomposite
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    rxcomposite Senior Member

    Please clarify.
     

  15. fallguy
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    fallguy Senior Member

    If there were any voids or low spots or pinholes; it sounds like you would prefer them filled with epoxy versus covered with paint? (I said resin, but it was 4:30am!).
     
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