spraying gelcoat

Discussion in 'Fiberglass and Composite Boat Building' started by 236eagle, Jan 14, 2015.

  1. 236eagle
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    236eagle Junior Member

    Newb here. I hope I'm in the right section. I have a 1.4 gravity feed spray gun and a 3.0. I tried spraying with my 3.0 spray gun. I did no thining with styrene or acetone. temperature in my garage was 74degrees.I am spraying a panel on the back of the boat are 10 inches wide by 6 feet long. I put on three coats one each day , I used a non wax white gel coat. I used way too much catalysts i know that for sure. the following is my issueand problem. When it cured II wet sanded it to get the orange peel out. I could never get the textured look out of the surface. It looked like there had been a bunch of small air pockets. I think you call it air porosity. Having said all this I think I need to learn from scratch the mixing and application with a spray gun. There was one horizontal place I sprayed, and was able to sand flat with no Orange peel or air pockets?. I was spraying 12" away from surface. Thanks so much in advance for any guidance
     
  2. gonzo
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    gonzo Senior Member

    If the surface below had a lot of pinholes, they may show through. Sometimes you can solve that by spraying really dry (from a farther distance) and after it gets tacky, spray wet. You should use wax in the gelcoat for a proper cure.
     
  3. 236eagle
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    236eagle Junior Member

    The last coat I used PVA thanks for reply. Wonder if there an article explaining the whole procedure?
     
  4. ondarvr
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    ondarvr Senior Member

    A book could be written on the potential causes of porosity, but here's a few.

    Too much catalyst.

    Wrong catalyst for that gel coat.

    Gel coat sprayed too thick.

    Gel coat sprayed too thin.

    Mist coating the gel coat.

    Gel coat sprayed too quickly.

    Gun held too close to the surface.

    Gun held too far from the surface.

    Air pressure too high.

    Air pressure too low.

    Contamination in the gel coat.

    Contamination in the catalyst.

    Contamination in the air lines or tank.

    High Humidity.

    Low humidity.

    Gel coat that is too old.

    Gel coat out of spec.

    Too much styrene evaporated out of the gel coat.

    Old catalyst.

    Spray gun not adjusted correctly.

    Sags or runs.

    Plus others.


    How much catalyst did you use?

    You could have sprayed all the layers in one session without a problem.

    Buy some Patchaid to reduce the viscosity, siphon and gravity feed guns have a very difficult time spraying unthinned gel coat. I don't normally recommend adding styrene because it degrades the gel coat, but adding a very small amount can help spraying it out of a small gun, it's a trade-off, better spraying for worse weathering.
     
  5. 236eagle
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    236eagle Junior Member

    OMG I'm really confused now. Lol
     
  6. SukiSolo
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    SukiSolo Senior Member

    Make sure you have Spraying gelcoat not just 'gelcoat'. The spray version is thinner and intended for that application. Whenever I have sprayed any polyester, I am always wary of the setting time of the catalyst - if the spray suddenly gets a bit thicker - run and clean the gun out before you have a fully set mix!.

    Just keep everything clean and dry, ensure no contamination on surface at all. Wipe with acetone first. Just so easy to get light contamination on the surface. Also might be worth the 'epoxy' trick, immediately after spraying wave a hot air gun over it to 'pop' any pinhole air bubbles, if these are still a problem. Personally, I'd be a bit wary of using a propane torch (fire risk is high) but a paint stripper gun/hair dryer might well be enough.

    I would also have a small % of wax (styrene monomer) as it self seals the coat to exclude air and allow full cure. Of course it needs cutting after but relatively minimal if you have a good coat. Good timing is your friend so if you get it right, not too bad.
     
  7. 236eagle
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    236eagle Junior Member

    Oh thank u so much. What air pressure? I have a 3.0 gravity hvlp gun. I use motor guard water filters
    thank u so much again! Gary
     
  8. ondarvr
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    ondarvr Senior Member

    Do not wave a heat gun over the gel coat right after you spray it, nothing good is going to come from it, it can actually increase the likelihood of porosity. 99.99999999999% of all gel coat sold in the US is for spraying, brushing is almost non existent here, so little of that version is produced.


    Pressures are determined at the time of spraying, every gel coat will be different, and ambient temperature will affect the viscosity. So pressures can vary.

    You never said how much catalyst you used.
     
  9. 236eagle
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    236eagle Junior Member

    Can you explain what is exactly air porosity? Thanks
     
  10. ondarvr
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    ondarvr Senior Member

    Porosity is just tiny bubbles in the gel coat film that don't come to the surface and pop prior to it curing.

    Anytime gel coat is sprayed there is air introduced into it, there are also air release agents added to help it escape. Tiny bubbles can be created from catalyst as the mix off gasses right after it's mixed too. So the correct methods of spraying need to be used to help reduce the amount of air in the gel coat as it's applied.

    If the air pressure is too low, the bubbles may be larger.
    Higher pressures can produce tiny bubbles.

    High viscosity gel coat makes it harder for the air to escape, so reducers are used many times when using gel coat as a paint, like in this case.

    Too much catalyst, or the wrong one can increase the chances of porosity too, they can create more bubbles.

    When using automotive type paints it's common to spray a mist coat, this is a big No No when spraying gel coat, it increases the chances of porosity. Gel coat has a certain amount of styrene in it, the levels have been lowered over the years by the Feds to reduce emissions. The lower levels of styrene are right at the limit to where the product may become unusable, if you use high air pressure, or mist coat, the styrene has a much greater chance of evaporating before it reaches the surface of the part. With less styrene the viscosity is higher and doesn't allow the air escape.

    High temperatures and low humidity allow more styrene to escape.

    Lower temps and higher humidity can increase the viscosity, which may make it harder for the air to escape.

    As you can see, finding the right combination of everything to get a good result can be frustrating at times. Most of the time everything goes OK, but occasionally there are problems, and identifying the cause can take a little work.
     
  11. 236eagle
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    236eagle Junior Member

    Thank u so much well said. I do auto. Painting so yes I fog on the first coat which u explain aND so no. I talked to Fiberlay and they say that they have a product with clear and I didn't ask him what clear ment. So what u r saying in regards to styrene, it is a good thing to use a measured amount. But if I keep adding coats one after the other, it seems that I would get solvent pop or I mean air porosity. So the time lag between coats is what I don't understand yet.
     
  12. SukiSolo
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    SukiSolo Senior Member

    Interesting, most gelcoat in the UK is for roller/brush application, in fact you need to buy 5Kg (11b) as a min for spraying!. Used to be 25Kg until quite recently. Whilst I would not apply a flame to sprayed gelcoat, I cannot see why a quick wave with a hot air blast should not help 'pop' any minor 'bubbles' IF they are present. Funny how I have recently run into osmosis on newish builds - and can see the brush marks....;) Not like the temperature is going to be derogatory to polyesester if under say, 100 Deg C and on fleeting only basis, and after some minutes to start to set.

    With respect, the chemical reaction (exotherm) is already started by virtue of the catalyst, all you are trying to achieve is release to atmosphere of any trapped air, hence a smooth coat. Why should polyester be so different to epoxy?. I've sprayed gelcoat with a really crude non compressor gun and it worked fine, 2K polyester primers too.

    Quite happy to accept some of the US stuff may be a tad different, so you may need to develop your own techniques to suit. That is life, a bit of trial and error to get there...;) Good luck finding settings that work, I'm sure you will arrive at them OK.
     
  13. ondarvr
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    ondarvr Senior Member

    Using a heat gun on the surface does a few things.
    1, Polyesters aren't as affected by heat when it comes to viscosity, they do thin down, but not as much.
    2, heating the surface causes the styrene to evaporate from the top layer, making the viscosity higher, this forms a barrier that the air will have more difficulty in passing through.
    3, Polyesters cure much faster than epoxies, so heating the surface can cause the top layer to gel sooner and seal it off so the air can't get through.
    4, it can also cause the bubbles to enlarge and may even create more.
     
  14. ondarvr
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    ondarvr Senior Member

    Gel coats, unlike paints, don't rely on evaporation for any part of the cure, it's just a chemical reaction, styrene is part of the reaction, not just a reducer, it has a dual roll. Adding more styrene (or anything) than what's needed can have negative results, but sometimes the tradeoff of easier spraying is worth the poorer weathering.

    All you need is a few minutes between coats before you can spray another layer. And when using it as a paint, you can let it gel slightly and then apply the next coat if you have thinned it to the point where it's starts to sag before you get it to the thickness you desire.
     

  15. 236eagle
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    236eagle Junior Member

    That is very true, need to get a test panel see what works. It's so nice for the input though at least it points me in the right direction
     
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