Again gelcoats, forever and ever

Discussion in 'Fiberglass and Composite Boat Building' started by oneilmemo, Sep 2, 2008.

  1. tinhorn
    Joined: Jan 2008
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    tinhorn Senior Member

    Indeed they do, rwatson. Vinyl less than poly, and it can be reduced somewhat by using fillers. I observed less shrink with microsphere-filled resin. Hehe - an acquaintance (not knowing I was in the biz) once showed off his new boat with "metal mesh buried in the fiberglass for toughness. You can see the pattern!"
     
  2. juiceclark

    juiceclark Previous Member

    Well, my boat is a 1982 F-36 Trojan with "flag blue" awlgrip. In person, you can clearly see the fabric in areas of the sides of the boat. The hull is 7 layers of glass below the waterline and 5 above with no coring.

    I hate the dark hull anyway - shows every damned drop of salt and dirt - but it sure looks good in pictures.
    [​IMG]
     
  3. the1much
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    the1much hippie dreams

    you guys dont think the reason you can see the fabric pattern is because the numbnuts that built the boats went to thin on the gel?,, like MOST boat builders do?
     
  4. tinhorn
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    tinhorn Senior Member

    Wouldn't matter, man. Resin shrinks - 'glass doesn't. The better the 'glass/resin ratio, the less shrink you'll see. Of course, with chopper guns, you'll always be resin-rich.

    Indeed - that boat is a beaut! Well, as long as it's perfectly clean, eh?
     
  5. ondarvr
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    ondarvr Senior Member

    Back to the original question first. The questions are very difficult to answer because you want specific answers to general and wide ranging questions.The chemistry can be very different between gel coats, so they aren't always compatable enough to just mix together (most times they are though).

    Adding different promoters, Inhibitors and using different catalysts can yield different results, some good, some bad.

    Gel coats do not exhibit all of the properties of the base resins they're made from. In being made into gel coat many other ingredients are added, some improve certain properties, some degrade others.

    Frequently more than one base resin is used to make a finished resin or gel coat, but you can't just blend any and all resins together. And this is only within a type of resin, not blending polyester with epoxy for example.
     
  6. ondarvr
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    ondarvr Senior Member

    As for shrink and print through. We all see the dreaded post cure on dark colors, it is from shrink, but there can be many causes for the shrink.

    Thin gel coat will show it to a greater degree, but is not the cause, the type of resin used, how it was catalyzed, the temperature it cured at and the resin to glass ratio are the main contributors.

    Some resins shrink a great deal, others don't, so a resin designed to yield a good surface profile will help.

    Using the correct amount of the correct catalyst to properly cure the laminte is very important.

    If the exotherm of the resin is too low it won't properly cure and it will continue to shrink for a long time.

    The higher the glass content, the more fiber print you will see, this has been a problem with infusion, the glass content is very high and surface profile can be poor. This can be overcome, but it takes more work than in hand lamination.

    Print blockers of differnet types can be used to help reduce the print.
     
  7. fiberglass jack
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    fiberglass jack Senior Member

    i find to help stop print through is to first skin the gelcoat with c-viel and one layer of 1.5 oz mat using vinyalester resin and make sure you put on a good 20 mils of gel, we all know you can get away with 12 or 14 mils but to stop print through you need a good layer of gel, the vinyalester skin will be harder then iso or ortho, and will be a good bairer for ossmosis
     
  8. ondarvr
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    ondarvr Senior Member

    The VE used for the skin coat needs to be designed for that purpose, most VE's are somewhat sluggish in the cure department and will continue to cure for a very long time after demolding. Skin coat VE's are highly promoted to cure well in a thin laminate, which can make them difficult to use for the bulk laminate thats applied thicker. Like f/j said veil and a good 20 mils of gel coat help a great deal also. There are several types of spray on print blockers and other veil type products too.
     

  9. naturewaterboy
    Joined: Sep 2006
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    naturewaterboy Steel Drum Tuner

    onelemon (or whatever your name is):

    you are looking for a gelcoat that you can put on some motor case that has to stand up to very high heat. There ain't NO gelcoat that will stand up to that temperature. I don't think there is any epoxy resin that will either. Sounds like you need a phenolic or some other material than what boats are built from. Any of the common boat building resins weaken really lots when they get a little hot - like at 100 degrees C, most resins will have less than 1/2 of their strength at 25 degrees (70 F). I'd say you need to look elsewhere for material - there are things like the cases of our grinders that are made from stuff like you are looking for.
     
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