Tooling gel coat wrinkled and blistered

Discussion in 'Fiberglass and Composite Boat Building' started by aaronhl, Oct 30, 2015.

  1. aaronhl
    Joined: Aug 2012
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    aaronhl Senior Member

    I have a plug that I brushed the partal 10 pva on, very shiny and flawless...I dont have spray equipment so I have to brush my tooling gel coat on. I brushed a layer of tooling gel on but it was a thin layer and could still see the plug surface through some on the fine brush stroke. I didn't see any blistering and about 10 hours later I put a second layer on. As I was finishing (15 minutes) I see some areas already wrinkling and blistering, which I didn't see at all when I put on the first layer. I check the mold the next day and most of the gel coat is wrinkled, which I can see holes through the 2nd and 1st layer to the plug. It was bad enough to have to rip the gelcoat off and will be starting over.

    I used 13 drops per OZ and the temperate was probably in the high 50s- low 60s in the garage overnight

    Why did this happen? I have used gel coat before and never wrinkled
     
  2. waikikin
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    waikikin Senior Member

    Styrens Attack!

    Hi,
    I think you are suffering from a case of tripe or "alligatoring" for the other side.
    Stop using "drops", you need some digital scales and an accurate measure for your catalyst- like a syringe, also give up on imperial measure and convert to metric weights.
    Your gelcoat needs 2%-2.5% catalyst.
    What has most likely happened is that particularly on the brush strokes the fibers of the brush have left the gelcoat thin then along comes the second coat and styrene within etches and melds into the already applied softening and wrinkling it......
    you need to apply thicker layer first up & if doing a second layer hit it once it passes the squeak test but not before... use your rude finger outstretched an drag with light weight... if it squeaks wait a bit then ok.
    You can also calc the area of the part, apply a "hang up" allowance for the pot and brush and arrive at a known quantity and theoretical thickness of gelcoat on the surface for a one hit operation.
    You need to work withing temps 16-30 Celsius pref about 20-22

    Jeff
     
  3. aaronhl
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    aaronhl Senior Member

    That information is very helpful.

    OK so since the first coat went on without any issues, the second layer burned up the first before hardening? And are use saying wait to put the second coat on once the first layer doesnt squeak when you drag your finger over to make sure the first layer is completely hard?

    The reason I like use the drops is because I only mix about 1-2 oz at a time. Do you have any tips for mixing very small batches?
     
  4. waikikin
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    waikikin Senior Member

    Use some digital kitchen scales, you can calibrate for accuracy against known liquid quantity(100ml fresh water should be 100 grams, 250ml should equal 250grams.... ml = milliliter, one liter should = 1000 grams or one kilogram), use the metric option for grams, these probably cost 20-25$, you can put in a plastic bag(unsealed!) to keep nice.... when you deliver back to kitchen;)
    Metric is tops for composite.... say you want 100 grams for the surface, you work out how much your brush hold at job end(maybe 30 gram) and also how much hangs back in the mixing pot(maybe 40 gram), keep some records for reference, you simply can weigh before & after, the difference is the allowance you make, so maybe you dispense 170- grams of gelcoat then use 2-2.5cc catalyst per 100 grams
    Get some syringes from the chemist- 5cc size is probably ideal for your small batches for the catalyst.
    Say you want to put .65mm thickness of gelcoat on the surface, you need about 750-800 grams of gelcoat per meter sqare to apply, so if your job is .5m x .5 m(500mm x 500mm) = .25 meter square you need 200 gram for the surface then the allowance for brush and pot... so 270 gram.
    different brush and pot will be different.. you need to work out... from your own technique and experience.
    Jeff.
     
  5. aaronhl
    Joined: Aug 2012
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    aaronhl Senior Member

    So the rule of thumb is 2-2.5cc for every 100grams?

    Since 4oz is approx 112grams,
    Each ounce should have .57cc? (2.25*1/4)

    I was also using a foam brush so I am going to try with a regular haired brush next and wait for a thick first layer to cure inside over a day.
     
  6. ondarvr
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    ondarvr Senior Member

    The failure of the first attempt was due to applying more gel coat over an undercured layer of gel coat, this is the cause of all alligatoring. The reason for the under cure can vary though. In this case it may have been from applying the first layer too thin, too little or too much catayst, or cold ambient temperatures.

    You can easily brush on enough gel coat coat and get the required thickness, it just takes a little work sometimes. Measuring can be easy, but most kitchen scales aren't accurate enough to measure small amounts of gel coat, as the amount increases their accuracy is close enough. You can use a small calibrated cup like what comes with Nyquil, or a syringe, they both work well depending on the amount you need to mix up.

    Wait times between coats should only need to be around an hour if the catalyst level, temperature and thickness are correct. No need to wait overnight.

    The gel coat needs to be applied at around 18 mils for a reliable cure, a business card is normally around 10 mils thick, so use that as a reference. Under 10 mils will frequently alligator when another coat is applied over it no matter how long you wait.
     
  7. rxcomposite
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    rxcomposite Senior Member

    Gel coat or resin is sold by weight or kilograms and polyester has a density of 1.2 kg/liter. MEKP has a lower density, cant remember the exact one but I know it is less than 1.0.

    For small batches, use volume measurements. The 2-2.5% by volume of catalyst is usually added to a liter of resin. It works for me. I use a plastic MEKP dispenser as the the O ring in the syringe quickly swell after some time.

    I usually do test on a half liter batch to find the best cure time for my need that is in accordance with the manufacturer's recommendation. After some time, I use substitute measuring dispenser like an old tablespoon or the cap of the MEKP gallon container.

    For your problem, you applied a second coat on a first coat that is too thin. (Also, when the coating is thin, you need to bump up the catalyst.) The second coat attacked the thin layer. Second, you waited a long time to apply the second coat. The squeak test by Waikikin works best. For me, once I touch the first coat without any color transferring to my finger is good enough. It should be cured enough but no more than necessary. Overnight curing is asking for problem. 1 1/2 hours to 3 hours cure max is my time.
     
  8. aaronhl
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    aaronhl Senior Member

    Great information guys, I think I am on the right track now. The first layer was very thin and I will need some practice to brush on a thicker first coat. And will only wait a few hours between coats

    I am in northeast United States and there is no way I can do this about like 65 degrees right now
     
  9. waikikin
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    waikikin Senior Member

    You can modify environment temp without much trouble, once your at 20C should go ok, just a small heater, also make sure the drum/can of gelcoat is up to temp, often sitting on concrete floor is bad, same for surface applying to as well.
     
  10. aaronhl
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    aaronhl Senior Member

    Right now I have a temporary setup in the garage, might consider a small portable heater with and enclosure around the mold.

    So if there are 30cc in an Ounce, adding 2.5% mekp would be .75cc right? 30/.025=.75cc Make sense to add .75cc mekp per oz of gel coat? I think I can get very accurate using syringes, rather than cups for the small amounts i mix (2-3 oz at a time)
     
  11. ondarvr
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    ondarvr Senior Member

    It's very hard to measure accurately with less than 100 grams even in the lab, there are some products that we QC at 50 grams, but it's only because the exotherm is so rapid and high at 100 grams.

    2 CC's at 100 grams is 2%, very easy to do.
     
  12. gonzo
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    gonzo Senior Member

    In the lab we measure to 0.0001 gram routinely. A medium quality kitchen scale can be accurate within 1 gram
     
  13. ondarvr
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    ondarvr Senior Member

    We have scales like that in the lab too, they are protected in an enclosure so the air movement doesn't alter the reading, they aren't practical for QC testing though. Kitchen scales often do read down to 1 gram, but we've tested the results on scales like this and they are marginal at best for small amounts of gel coat. You need to go to hundreths of a gram for best results, tenths can be OK on some scales.

    If it only goes as low as 1 gram you may be at .8 or 1.8 and still read 1 gram, that's a huge variance when you want to catalyze 100 grams at 1.5% (1.5 grams) let alone 50.

    If you are measuring 1000 grams of gel coat and 10 to 20 grams of catalyst a kitchen scale works fine.

    We use 100 grams of gel coat for testing in the lab because it's accurate and easily repeatable when doing it 50 to 100 times per day. We've tested 50 grams and the results aren't as reliable.
     
  14. ondarvr
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    ondarvr Senior Member

    Not sure how this got here
     

  15. aaronhl
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    aaronhl Senior Member

    OK just used some syringes and it makes for easy cleanup and measuring, but I am measuring by volume and not weight (going to get a scale soon)

    I measured 90ml gel coat (approx 3oz) and 2.25ml mekp which is 2.5% by volume in the syringes

    I also used a brush with hairs this time and patted the gel coat on instead of brushing it on. I am going to do one thick layer this time, until I can get some practice with thinner layers now that I know exactly what alligators are (thought we didn't have any up here in CT!!)
     
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