Another Gel Coat Question About Alligatoring....

Discussion in 'Materials' started by Sabre1FST, Sep 8, 2023.

  1. Sabre1FST
    Joined: Sep 2023
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    Location: San Jose, Calif

    Sabre1FST New Member

    First of all I have read many of the other threads on Gel Coat alligatoring problems, but not all of them... To be honest, I am making composite parts for a race car not a boat, but in searching the net, I have found so much more useful information here than anywhere else, which is why I am hoping the knowledge base here can help ;-)

    My process is as follows:
    1. Spray PVA on mold surfaces in multiple passes to get a good uniform layer
    2. One day later, spray catalyzed Gel Coat with a pressurized spray gun; usually diluted 10% with Styrene monomer. MEKP dosage is based on ambient air temp, usually 5-6 drops per ounce - I am looking for 1 hour of working time which gives me time to clean up spray gun before Gel Coat starts to solidify; humidity is not an issue here. Gel Coat is applied in multiple passes - 2 misting coats, wait ~ 5 minutes, then 2 more misting coats, etc. until all the material is applied.
    3. One day later, coat part surfaces with polyester resin with paint brush.
    4. One day later, start lay-up with .75 oz. matte and then 2 layers of cloth with occasional use of rollers to work out bubbles.

    Randomly it seems I'm getting the alligatoring of the gel coat after I have completed step 4. I know that too thin a layer of Gel Coat is a common cause for alligatoring, but on complex shapes it is difficult to apply a uniform Gel Coat layer; I have the film thickness gage, but have not tried using it, yet. I will calculate part surface area, multiply by .020" (desired film thickness) to get a required volume then add a small amount for losses (overspray, residue in the gun, etc.). Usually this works well, but recently this problem has reared it's ugly head. The air supply is filtered for water, oil and particles so I don't think contamination is an issue. Gel Coat and resin are fresh material < 2 months old. The molds do not trap the styrene vapor so that isn't an issue. It seems that the gel coat is softening/dissolving when the polyester resin is added and using a roller seems to accentuate the dissolution. I'm wondering if waiting more than 1 day between Gel Coat and polyester resin application will make a significant difference in Gel Coat stability; or possibly there are variations in manufacturers formulations that might make one brand better than another. I have asked the manufacturers tech support about this and haven't gotten much help. Your thoughts?

    Thank you for taking the time to read this and any useful suggestions.
  2. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    You seem to be building in a lot of weight and taking a long time to do it.I suspect your mix is under catalysed if the resin is softening it and this may be occurring because you feel a need to use a slow mix so that there is time to clean the spraygun.I have no idea what your catalyst ratio is since we would normally express it as a percentage based on how many grams are involved and 2% is quite typical.It would be a huge amount faster if you used wax instead of PVA and left the spraygun on the shelf as you could apply the gelcoat by brush within minutes of the final wax.It would also save on mixing quite so many chemicals and cleaning the spraygun.You would then be able to begin applying mat as soon as the gelcoat has cured and certainly within a couple of hours.Why you choose to apply a coat of resin with no reinforcement is a total mystery as I have never encountered this technique so skipping this part would gain a lot of time and save weight..Use of gelcoat on most racing car parts is a means of giving less colour difference when the time comes for painting and they do tend to get a final coat of paint,both when new and when minor damage has been repaired as this is the time to bring out the spraygun.Paint tends to have a higher pigment concentration than gelcoat and this also helps with keeping the weight down.Keep in mind that high end racecars are moulded from autoclaved prepreg and no gelcoat is used.
    Tops likes this.
  3. Sabre1FST
    Joined: Sep 2023
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    Location: San Jose, Calif

    Sabre1FST New Member

    Wet Feet,

    I appreciate your reply but it doesn't address my question.

    The manufacturer recommends 2% MEKP for the Gel Coat which is 14-16 drops per ounce of Gel Coat, this then provides 10 to 15 minutes of working time at 77 deg. F. This is not enough time to spray multiple molds, multiple times with a 5 minute pause between each spray application. I have opted for roughly 1% MEKP which gives me about an hour of working time which is adequate for what I am doing. The manufacturer does recommend a minimum of 3/4% MEKP. After a couple of hours, the sprayed Gel Coat surface is fairly hard and can barely be scratched with a fingernail, which seems like it should be adequate for laminating, but I give it 24 hours and then apply the layer of resin to provide a barrier layer to hopefully minimize/prevent dissolution of the Gel Coat during the lamination process. I do wax the molds and wait a day before I spray the PVA. I tried using wax only, one time. I was unable to separate the part from the mold and gave that mess and the wax back to the company that sold it to me (they had given me detailed instructions on how to use the wax prior to my using it, which I followed); though I was tempted, I did not throw it at them. The wax I use and the PVA have been in use for decades for this application; I find the PVA is the primary release agent and the wax is a marginal secondary barrier. I have never had any success using a paint brush to apply an even layer of Gel Coat to a mold.

    It seems the rules are very different when you are applying Gel Coat as a final layer over a laminate (such as a boat hull repair) versus being the first layer that is then laminated over (as in a mold) - yes of course, some (many?) boat hulls start with a Gel Coated mold that is then laminated and I'm hoping to hear from someone that has knowledge of that process or other relevant experience.

    As for the prepreg and all the equipment required to use it properly, yes it is better/lighter/stronger/faster/more expensive/makes you lose weight (through all the extra work you have to do to make it work) is nice but not what I'm asking about.
  4. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    Let me phrase it a bit differently,if you are compromising the catalyst ratio to suit your preferred application method,don't be surprised if there are consequences.The application of a coat of resin is introducing a lot of styrene to an imperfectly cured gelcoat surface and as you use styrene to thin the gelcoat it should be apparent that the styrene is quite capable of reacting with the gelcoat.The obvious options are to learn to use a brush to apply the gelcoat quickly and adequately or to forgo the gelcoat and paint the component.
    BlueBell, kapnD and Blueknarr like this.
  5. KD8NPB
    Joined: Mar 2018
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    Location: South Carolina

    KD8NPB Senior Member

    Initiator (MEKP) should be measured by ML or by weight, not by drops. Minimum 1.5 or 1.75% is typical.
    Minimum application wet thickness is usually 12 mils with 18 mils being the target.
    US origin gelcoat is always formulated for spray use, not brush.
    It should never be diluted with styrene.
    PVA application, gelcoat, and skincoat should all happen same day.
    Skincoat should occur within 4 hours of gelcoat application
  6. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Let's start over.

    Skip everything you heard or thought you knew before finding this site, then read what members here have to say, they are very experienced.

    There is only one reason for alligators, and it's under cured gel coat, now there can be many reasons for it to be under cured, but that's the reason. Under catalyzed, over catalyzed, cold temperatures, not waiting long enough, styrene fumes collecting in low areas and thin gel coat are the most common causes.

    There is no accurate way to counts drops of catalyst, you either need to accurately weigh it, or measure by volume. And the smaller the amount you mix, the more critical the measurement's accuracy needs to be.

    There is no gel coat manufacturer that recommends 1% catalyst, 1.5% is the minimum, 1.8 to 2% is typical.

    Don't add styrene, if you can't spray it as is, get a different tip size (larger) or a cup gun. You can also try brushing and/or rolling, but it's a bit tricky until you figure out how to do it right.

    Resin coating the gel coat is worthless, so skip that step.

    Spray the areas on the mold you can, and use the mil gauge to ensure the correct thickness. If there are thin spots you can try pre brushing those spots, or brush gel coat on after the gel coat is hard enough. Depending on the exact part and other aspects one method might work better than the other.

    You should apply the laminate as soon as the gel coat doesn't transfer to your finger. Well, that's the official answer, and the best one, but it's not always possible. But there's no reason to wait any longer than the minimum reasonable amount of time. Out of convenience some people will spray in the evening and laminate the next morning, which is typically fine, but don't wait days.
    fallguy likes this.
  7. waikikin
    Joined: Jan 2006
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    Location: Australia

    waikikin Senior Member


    Everything Ondarvr said and this, .. don't mist it, lap pattern with a cup gun, they wash out quick with a pot of acetone, use what hangs up in the cup to carefully brush in any spray shadow areas. If you want to extend open time after the gel but cant laminate same day use surface veil, your resin coat is not helping.
    .... here's another trick & not joking, you know your "rude finger" well, with gentle pressure swipe that on a flange edge or whatever, if it comes up greasy with gel then wait, if it "squeaks" then get to it and laminate:).


  8. Cobra1
    Joined: Feb 2008
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    Location: Az

    Cobra1 Junior Member

    You are doing it wrong. For one trash the pva, use mold wax. Meassure the MEKP, no one goes by drops. One hour working time is no good, you want that stuff mixed properly, 20 to 30 minutes working time. Alligator gel, the gel is to thin, 15 mill minimum, 20 is better. It sould only take 2 minutes to flush a pot and gun, cup gun 30 seconds, a brush, put that in the trash.
    once the gel has kicked, start laying glass, no need to waiy a day.
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