two tone gelcoat application

Discussion in 'Fiberglass and Composite Boat Building' started by jiggerpro, Oct 29, 2014.

  1. jiggerpro
    Joined: Sep 2007
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    jiggerpro Senior Member

    Since gelcoat, to avoid microporosity must ideally be applied in three layers first 0.4 mm thick plus two others at 0.2 mm for a total of 0.8 mm, I do not see how can this be made when making for example a hull with the sides in blue and the bottom and transom in white

    I understand, that the masking tape has to be removed while gelcoat is wet but, since gel coat should be applied in three layers first layer 0.4 mm and when dry another of 0.2 mm and when dry, another of 0.2 mm for a total of 0.8 mm, this implies that after everyone of those layers is applied the masking tape around the blue colour border has to be re-taped, since this can not be done absolutely perfectly it means that there will be small narrow areas around the first masked area where there will not be the 0.4 mm thickness necessary to prevent alligatoring, so I do not see how this can be done in an absolutely guaranteed way where luck does not play a role.

    The only way I can see this being done successfully is to apply 0.4 mm of the darker colour (ice blue on the sides for example) , remove the masking tape while wet and afterwards when cured, apply the lighter colour (white) in 0.4 mm thickness to the bottom and when cured apply two layers of white 0.2 mm all over the piece, the only inconvenient would be then that in the sides (darker colour) then the thickness of the ice blue on the sides would only be 0.4 mm plus two layers 0.2 mm in white backing it, I do not know if this is normal procedure or if it would not be a good quality job because ideally, the blue should have the total thickness of 0.8 mm and not just 0.4 mm, can someone give some expert advise about the best way to accomplish this ??
     
  2. rwatson
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    rwatson Senior Member

    Method two is what I have seen done only. The 'thin' color is still thick enough for structural requirements, but if a deep scratch reveals the underlying 'thick; ( white ) colour, you just have to try and match the repair as best you can. ( never easy with gelcoat )
     
  3. rxcomposite
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    rxcomposite Senior Member

    You dont need to spray two layers of gel coat as the thickness can easily be controlled by the gun. Use a wet thickness gauge which you can get free or a few cents from the gel coat supplier. Porosity is not an issue with a good quality gel coat.

    Spray the first dark coat, remove the tape, and spray the next gel coat with slight overlap on the dark side. Make a trial pass first before commiting to a large piece. That way you would be able to gauge when to remove the tape and when to spray the 2nd color. If the first coat is too dry, you won't get a clean line. Too wet and it smudges.
     
  4. rwatson
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    rwatson Senior Member

    When the OP said 'apply', I envisaged hand application. Would the multiple layers be applicable in that case ?
     
  5. rxcomposite
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    rxcomposite Senior Member

    "Apply" can be by brushing, roller, spray.;)
    By the way, I use a very sharp shaving blade just in case the tape lifts the semi cured gel coat. Comes in handy.
     
  6. rwatson
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    rwatson Senior Member


    Sure - I understand that - but the questions was "do you need to do it in multiple layers if you apply it manually ? "

    I seem to remember that it is the preferred way to get a minimum reliable depth
     
  7. rxcomposite
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    rxcomposite Senior Member

    You are correct. One pass, use wet film gauge for accuracy.
     
  8. jiggerpro
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    jiggerpro Senior Member

    Hi Mr Watson & Mr rxcomposites,

    we plan on spraying everything, and we have airless gear to do that, in regards to the application of second layers (after application of the first blue 0.4 mm coat) I have seen some builders that say that after the blue colour layer was aplied, quickly, while the gelcoat is still wet the masking tape should be removed then it should be left to fully cure for aproximately 12 hours before aplication of the next layers of the white colour, Mr rxcomposites, this is slightly different from what you suggest and I can not see why if the first blue layer was completely dry the line between the two colurs would not be a clean one, maybe you can explain this a bit
     
  9. rxcomposite
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    rxcomposite Senior Member

    From experience, if I let the first gel coat to fully dry, there is a tendency for the second pass to soften the fully cured gel coat causing mottling or even lift off. This is also true when applying the 1st layer of laminate. With a fully cured gel coat, the reinforcing laminate softens the gel coat and cause popping up (wet gel coat runs under) or premature delamination.

    As a rule, the longest time we can leave a gel coat is 8 hours on a cold night. Best adhesion is when the gel coat is "tacky dry", which means if you touch it with your finger, it is a little bit tacky but the color will not transfer to your fingertip. This is about 1 hour from the time it was applied. This is as recommended by the manufacturer. Even Class rules requires that laminating is a continous process. That is the 2nd layer is laid on while the 1st layer is still tacky. If cured, sanding is required.

    As I have explained, remove the tape (by doubling back) on the first coat while it is wet to tacky dry, wait until it cures more a bit then apply the second coat. It works for us but takes some skill knowing when to take the next step.
     
  10. jiggerpro
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    jiggerpro Senior Member

    Ok, thanks for your time and expert advise ..
     
  11. ondarvr
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    ondarvr Senior Member

    While techniques differ in different parts of the world, letting each layer cure before applying the next is not the norm, or recommended way. When it says three separate passes, it refers to spraying one pass, letting it gas off for a couple minutes and then applying the next pass, repeating this until you get the desired thickness. None of these passes are allowed to cure before the next pass is applied.

    When spraying a boat hull you can spray a large area, possibly one side of the hull, or even the entire hull depending on the size, then immediately start spraying the second pass, the time that it takes to cover a 10' to 15' section is typically long enough that a second layer can be applied right away. The exact pattern and method will be different on a 16' boat compared to a 116' yacht, but there is no waiting for each pass to cure before applying the next one.

    The tape is pulled shortly after the final pass is finished and before it starts to gel, the exact timing depends on when that particular gel coat is at a stage where it won't sag, but doesn't develop strings that stick to both the tape and part leaving a ragged edge.

    The color that is sprayed first is determined by it's location and area it covers, not by the actual color. Normally the color covering the largest area is sprayed first. The timing of spraying the second color can vary, it may be sprayed right away over the first uncured gel coat, or you can wait for the first color to cure. The timing varies due to size of the part, temperature, type of equipment, personal preference, etc.
     

  12. jiggerpro
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    jiggerpro Senior Member

    Thank you Mr Ondavr for your very clear and complete explanation, I believe that I have fully understood it now, unfortunately so many different things are written that is difficult to know what to do, but what you say makes ( a lot of ) sense to me.
     
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