Sanded through gel coat on mold, What are my options?

Discussion in 'Fiberglass and Composite Boat Building' started by killarny123, Sep 14, 2016.

  1. killarny123
    Joined: Sep 2016
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    Location: Ontario Canada

    killarny123 New Member

    So Ive created a mold using polyester tooling gel coat and 6 layers of 1.5oz mat but I accidentally happened to get a few air bubbles in the laminate that turned to holes in the gel coat. After filling the sanding those spot's level I noticed i sanded down to where i can see fiberglass coming through the gel coat. What are my option's for repairing these spots, can I just polish the mold as is and use it for an infusion layup process or will the fiberglass cross link and bond, or should I respray another coat of gel coat onto the mold? Any help is greatly appreciated.
     
  2. ondarvr
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    ondarvr Senior Member

    Either way will work, the surface finish will be better if you repair it with gel coat though.

    The gel coat and resin used in the laminate share the same chemistry, so if it was going to bond where you sanded through, it will bond to the gel coat in the same way.
     
  3. guam2250
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    guam2250 Junior Member

    I have the prototype Knickerbocker One Design, 1960 built. It was a one owner by a friend of Olin Stephens. While it is in good shape, and been covered and out of the water for many years, there is an area just above the waterline, in the aft overhang, where the gelcoat has come off in a piece about as big as my hand. The inside of the gelcoat piece and the glassed hull underneath are completely smooth and don't look they ever adhered to each other. This was an early fiberglass hull. Anyone have any ideas what caused this? If the whole hull is going to do this, I'm in trouble!
     
    Last edited: Sep 14, 2016
  4. Tiny Turnip
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    Tiny Turnip Senior Member

    I've always understood that if the surface of the resin is in the open air as it cures, then flowcoat should be used rather than gelcoat.

    I believe that the air slightly inhibits the curing chemistry, and so gel coat cured in the air will remain slightly tacky.

    In this instance you should use flowcoat, which has a wax additive. The wax forms a film on the surface of the curing flowcoat, keeps the air out and gives you a properly hard cure.

    I'm absolutely no expert though; perhaps others could confirm one way or another?
     
  5. SukiSolo
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    SukiSolo Senior Member

    Close Tiny. Gelcoat itself requires the addition of Styrene Monomer (Wax) to let it cure without another sealer from oxygen (air) such as cling film. The wax rises through the gel to the surface and seals it allowing full cure. You can add more but you MUST abrade the wax from ythe surface to get a good bond.

    If your making a full moulding, you don't use the wax (additive) because the first layer of he laminate does this for you and you get a sound chemical bond.

    Always check whether any gelcoat you buy has the Styrene Monomer added or not. It often is, in small repair packs. The addition of the wax significantly shortens the storage life of gelcoat btw.

    ondarvr is perfectly correct, well said sir.
     
  6. guam2250
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    guam2250 Junior Member

    Any ideas as to why some of the gelcoat did not adhere to the fiberglass hull on my Knickerbocker?
     
  7. ondarvr
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    ondarvr Senior Member

    Best to start your own thread so this doesn't get confused.
     
    Last edited: Sep 16, 2016
  8. redreuben
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    redreuben redreuben

    Close SukiSolo, Styrene monomer is a solvent and comes waxed or unwaxed. But yes wax needs to be added to the tooling gelcoat when applied to mould repairs. The wax is just paraffin wax and can be added to neat styrene monomer.
    The diy method is to buy a plug of paraffin wax and shave it with a cheese grater or similar and add it to warmed styrene and stir till dissolved by placing the styrene container in a warm water bath. Sorry, can't remember the ratio.
    Another method for spot repairs is simply to cover the wet repair with tape or glad wrap until it cures.

    The mould will be ok if just resin is exposed although it will degrade faster than the tooling gel if you are taking lots of parts, if fibre has been exposed it will be ok for a one off part but you will need to refinish the part to get an even gloss on the part.
    Flocoat could be used as a quick repair but it is full of filler and will most likely go dull and degrade faster than tooling gel.
     

  9. SukiSolo
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    SukiSolo Senior Member

    Cheers, redreuben.

    With wax filled Styrene Monomer, the normal raio is 2%, same as hardener. The key is temperature as below 10/12 deg C it just stays too thick to work into the gelcoat properly. Easy in summer but in winter I often locally heat it up. Fwiw for anyone doing this stuff, mix the wax in prior to the hardener...
    It's just easier to add the liquid rather than grate a paraffin block!, but your quite right. The ratio of paraffin to SM in the latter, I don't know offhand.

    Almost all the UK S M is waxed afaik (hence the bracketed wax before) and shelf life is still pretty short, nominal 2 months....
     
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