sanding between layers?

Discussion in 'Fiberglass and Composite Boat Building' started by member 35191, Aug 23, 2010.

  1. member 35191

    member 35191 Previous Member

    Hi, I am currently practising the lay up process on a square piece of wood. Before I start mt project of giberglassing a wooden banana boat.
    So far I have applied 4 coats of mat resin sanded it down smooth and cleaned the surface with acitone. Today I applied a coat of gel coat, this was my first time and while I tried my best to make it as smooth as possible it still looks abit rough. What do I do once this layer has hardened? Do I sand it smooth again before applying the next coat of gel?
    Edit/Delete Message
     
  2. apex1

    apex1 Guest

    You are doing it the wrong way round.

    The gelcoat is used only when you make your layup in a female mould.
    On the outer surface it makes no sense. Fairing and painting is the way to go here.

    And 4 times mat does not provide much strength, there should most probably be some fabric in between!??
     
    Last edited by a moderator: Aug 25, 2010
  3. member 35191

    member 35191 Previous Member

    You mean a fabric in between the mat layers? What would that be? Cheers
     
  4. apex1

    apex1 Guest

    No,

    I mean a fabric instead!

    Mat does nearly nothing to make a strong hull. Multiaxial, or biaxial, or woven fabric is what you want after the first and possibly second layers of mat on the gelcoat.
    Here the mat mainly prevents the print through of the fabric pattern.

    This is a example of simple woven fabric:
    [​IMG]

    simple mat:

    [​IMG]

    Here a "unidirectional" oriented fabric:

    [​IMG]

    a coarse twill:

    [​IMG]

    and this shows a combination of roving and mat:

    [​IMG]

    bidirectional:

    [​IMG]

    hope that helps.
    Richard
     
  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Mat is good as first bonding layer. Structurally, woven or stiched fabrics are much better. There is no need to sand in between layers. If you are using polyester, laminating resin stays "green" (the surface sticky) so you can add more layers and get a good bond. Fairing compound and paint is the easier way to finish
     
  6. tinhorn
    Joined: Jan 2008
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    tinhorn Senior Member

    Kind of.

    When a layer of mat has cured, there will be fibers sticking up. If these fibers aren't knocked down with a quick once-over with coarse paper, they'll create little air bubbles when the next layer is applied.
     
  7. gonzo
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    gonzo Senior Member

    Laminating resin is air inhibited.
     
  8. tunnels

    tunnels Previous Member

    The gel coat you are refering to is the finishing coat ?
    Gel coat does not behave like paint it is thick and will not flow out like paint !! But you can help this a little by adding some laminating resin to the gel and that will make it flatten a little better and it is still sandable when it has all gone hard !!:D
     
  9. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Hi John,
    for finishing off you need what is called flowcoat- it's pretty much gelcoat with a wax in styrene solution added, with care you can get a good finish but it takes some work, it can be sprayed or applied by brush- tipping carfully in a few directions sequentially, to get it smooth & shiny some work with wet sanding & cutting compound- if you google surfboard building the techniques may be better described & illustrated. All the best from Jeff.
     
  10. Wynand N
    Joined: Oct 2004
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    Wynand N Retired Steelboatbuilder

    you are quite right. If a layer had "kicked before the next going on, we also give it a once over with sandpaper for that very reason.
    The fibre will keep the laminated wetting out directly over it and result a little conical "tent" or air pocket, Had them many times in the past when an operator neglect to do his job properly.
    The give away for this is a dry white spot of mat that refuses to wet so to speak.
     
  11. apex1

    apex1 Guest

    May I repeat?


    John,
    Leave all the gelcoat nonsense, you are sheathing a wooden boat, not producing a GRP boat in a mould.


    Tinhorn,

    the question was if the gelcoat layers should be sanded! There is no glass standing proud of the coat.

    Regards
    Richard
     
  12. thill
    Joined: Mar 2007
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    thill Junior Member

    When doing a big transom repair, I used unwaxed gelcoat first (rolled on) then once it gelled, but before it cured fully, I knocked down any imperfections wth an orbital sander, blew, and then rolled on waxed gelcoat.

    Next day, I used 220 grit in the random orbit sander, followed by 600, followed by 1000 grit.

    Then I switched to 3M fiberglass restorer with a buffing pad, and you couldn't find the repair. Beautiful and shiny!

    I'm no expert, but it worked extremely well for me.

    -TH
     

  13. GG
    Joined: Jan 2008
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    GG offshore artie

    Keep it plain and simple spraying the jell with a gun that will push the material thru it thinning it with acetone going with more mills then usual because it all gets sanded and sprayng PVA over the jell instead of using a wax addtive which is very old school .
     
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