Epoxy Thinning for Outermost Layer

Discussion in 'Fiberglass and Composite Boat Building' started by RochesterSS, May 8, 2011.

  1. RochesterSS
    Joined: Feb 2011
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    RochesterSS Junior Member

    Hi, I am building a composite boat of half inch divinycell as the core with 3 layers of fiberglass on the outside and 2 on the inside. I talked to someone at my school and he said we need to go over the outer surface with more epoxy to smooth it out. So far the boat has three strong layers of fiberglass on the outside, but we are running low on epoxy, having used half of it and we still have half the fiberglassing we need to do, not including the smoothing layer.

    Can we use some sort of thinner to decrease the amount of epoxy we need for the smoothing coat? If so, how much? We are looking to do this on Monday when the rest of our epoxy arrives.

    Thanks,
    Rochester Solar Splash
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    No, you should never thin epoxy. For fairing, there are fillers already available. Depending on whether it is above or below the waterline and expected immersion times, different additives would be required. Tell your suppliers what is the intended use.
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Adding balloons, silica and/or spheres to the epoxy will make a fairing compound. This increases the volume of the epoxy proportionately. There's no substitute for goo. If you're short, you're short and have to get more.

    As Gonzo said, don't thin epoxy without a chemical engineering degree to back you up, as just the slightest solvent dilution, will make huge differences in the physical attributes of the epoxy.
     
  4. tunnels

    tunnels Previous Member

    Who ever gave you the idea to thin your resin needs a good kick !You never thin resins ever !!! :eek:
    By thinning you are completely defeating the purpose of using the stronger epoxy resin as it will have lost more that 1/3 to 1/2 its strength . For a fairing compound Q cells mixed with the resin to a consistancy where it is trowelable without slumping on a vertical surface if you mix slica with it as as well it gots hard to sand and will become brittle .:D:p:p
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Silica with Q-cells (or other light weight filler material) will not make the cured resin brittle, unless you use mostly silica and a little Q-cells. Silica is used to control viscosity in fairing compounds and in this capacity, you need very little to greatly effect the thickness of the goo.
     
  6. tunnels

    tunnels Previous Member

    Q cell by its self is brittle !! Silica by its self is brittle !!. two mixed together = brittle !!
    Be it polyester resin ! , vinylester resin ! or epoxy resin !! still comes out brittle !!!:?:
    Only exception is the thickness of the layer that you apply !, a thin layer is less likely to crack , the thicker the layer the more likehood to cracking if there is any movement in the panel !!

    I just been through all this where i am working , great globs of resin and silica mixed to a stiff blob and using it to hold things together and wondering why if it gets a knock it breaks the bog and comes aparts . Add a little micro fibre then things change but that defeats the purpose and makes it hard to workm with .
    The amount of filling these guys do i dare not introduce then to Q cells !! But add a little micro fibre or chopped strand glass and the whole senario changes .
    Sorry been seen and had to explain and rectify this on many occassions in quite a few places ive worked over the years . :D:p:p
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    No one is suggesting an all silica or Q-cell mixture. As a rule silica is used sparingly, to control viscosity and add stiffness to a mix. Most folks don't make very good filler mixtures. It would be rare for me to use one or two materials in any mixture. A straight silica mix, to that of peanut butter would reduce the cured resin tensile and compressive strength by about 20%. It's modulus of elongation would degrade as well, but I don't have figures for how much. This is why you always add "fibrous" materials to structural mixtures, to gain a physical interlock within the matrix.

    If you mix Q-cells into the resin until it's about like runny catchup, then add enough silica to stiffen the mix to peanut butter, you should be fine for a fairing compound on vertical surfaces, though I would still add a splash of talc to make a smoother mix, possably some phenol balloons. In fact phenol balloons and a little silica is exactly what West 407 is. My preferred stiffener is cotton flock, which is quite fibrous and also the material in West 403. To be honest, I don't have much need for Q-cells.
     
  8. CatBuilder

    CatBuilder Previous Member

    PAR nailed this one.

    You must do exactly what he said. Use a mixture of microballoons (or Q-cell) and colloidal silica. A bonus there is your epoxy will go farther. Mix it up just like he said smear it on like you would joint compound on sheetrock. This will fill in the weave and make a more smooth surface.


    Then, just a light sanding will give you a very smooth finish.
     
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  9. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    How would adding thinner make more epoxy ?

    Does two cups of boiling water make more coffee dissolved in the brew ?
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You can thin epoxy, which does make the quantity of the goo increase, but you really need to know what you're doing and what chemicals to employ. Other then formulator chemists, their tech support people and a couple of engineers I know, I've only met one person that actually understood the subtleties of the DEBA molecule.
     
  11. rwatson
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    rwatson Senior Member

    yes, you can thin under those implicit circumstances, but it wont increase coverage unless the solids are increased.

    Say I can get 100% cover of a hull at .5mm with a thinnned formulae, or 1mm cover over half a hull.

    If the depth of the solids isnt enough to cover the weave, I will either get a whole hull with half the weave exposed, or half a hull with a fair surface.

    Thinners dont add substance after a cure.
     

  12. tunnels

    tunnels Previous Member

    If you are looking for a smooth surface use peel ply !! No adding fillers etc etc :D
    When i glassed my son's boat used peel ply !, ripped it off and then undercoated sanded and top coated no fillers required
     
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