Spraying gel coat

Discussion in 'Fiberglass and Composite Boat Building' started by makonate, Feb 22, 2014.

  1. makonate
    Joined: Feb 2014
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    Location: rockport ma

    makonate New Member

    I am a rookie to gel coat but am trying to by my own set up so I can repair my 17 and 19 foot makos. I am looking for any tips on what size compressor and at what scfm I should be spraying and also what I need for continuous spraying? Any help or suggestions would be great.
  2. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    I'm not sure of the intricacies of spraying gel coat, but for paint you want at least 10cfm (12cfm is better). I don't see why gel coat thinned to a similar consistency would be much different - different sized spray nozzle maybe? You might want to consider one of the 2-pack epoxy marine paints like the ones made by Jotun (I have used the Imperite one) and at least some of them can be sprayed with an airless setup. The benefit of an epoxy or 2 pack polyurethane based coating would be a more waterproof coating that may be a fair bit easier to apply than gel coat, certainly less sanding/polishing and you get more time to get to the next coat without dust or the need for extensive sanding ruining your weekend.

    If you do spray gel coat as a top coat, make sure you get the right stuff - the spray stuff not hand layup. Most gel coat comes as air-inhibited as it is usually the first layer in a mould and will not completely cure until you add the next layer of fibreglass effectively isolating it from the air. That works for all but the last coat of your finish. Your final coat should be "waxed" gel coat or flow coat. Flow coat is gel coat that has a wax in styrene additive (you can buy "wax in styrene" to add to your inhibited gel coat to make it "waxed") which means that the wax will move to the top and form a barrier to the air and allow the flow coat to cure without a sticky surface - it has a waxy surface instead. Obviously, dust settling onto your hull between coats is your enemy as it will stick readily to the tacky gel coat and possibly ruin your finish.

    As I mentioned before, I haven't ever tried to spray gel coat or flow coat, so you can take my advice with a grain of salt there, but I have used a bit of the 2-pack epoxy stuff. You can even apply that using the "roll and tip" method and get a finish like a sprayed finish providing you have 2 people - one rolling the paint on and the other lightly brushing out the pattern left by the roller. This is the low-tech, low budget technique that might work better for you if you have a workshop where an air compressor would stir up too much dust.

  3. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    CFM depends on the type of spray gun and tip size you plan to use.

    A siphon or gravity feed gun will use more air than a pressure pot. The gun you choose will have CFM recommendations with it.
  4. pescaloco
    Joined: Feb 2006
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    Location: so. california

    pescaloco Senior Member

    You will probably want a compressor with at least a 60 or 80 gallon tank

    for what you are going to do a HVLP gelcoat spray gun with a 2.5mm tip will work great

    You can use a smaller compressor if you need to but if you have to buy one might as well get one that will be big enough if you do some serious spraying or want to run air tools
  5. SukiSolo
    Joined: Dec 2012
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    Location: Hampshire UK

    SukiSolo Senior Member

    Biggest problem I found was that Spray Gelcoat is only available in 25Kg tubs. Now with a 9 month nominal shelf life, there is no way I can get through it fast enough for what I do. For new build it is worth it, but repair shops etc will find it hard to get through the quantity to justify the waste.

    I have sprayed 2k polyester primer/undercoats which are catalysed and pretty thick. With a sufficiently open nozzle they are fine. Just be aware of the curing going on in the gun. If the flow rate drops whilst spraying, you are probably getting curing going on in the paint/gelcoat reservoir!. Even quite a few builders use rollers instead, judging from close inspection when repairing damaged areas on dinghies, small keel boats etc.
  6. leaky
    Joined: Sep 2008
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    Location: nh

    leaky Senior Member

    Start with a cheap spray gun. My attempt at this last year resulted in death of the spray gun (went off too fast)..

    I also was having a lot of trouble with a 2.5 tip. Now another friend of mine, who actually knows how to paint ;), gets good results with a 2.0 tip.. Another painting expert I know claims it's impossible without a 3.0 tip.

    It may come down to temperature, which has a big impact on viscosity. You can thin gel coat (read instructions, some acetone, some xylene) - but it can have an adverse impact on curing or even the properties of the finished product.

    This stuff is something to look into, at least for the last coat:


    As the amateur that I am, I've given up on trying to learn how to spray gel coat myself - if I just want gel coat I use a foam roller and then will sand/buff to get it to a nicer (but not perfect) finish. If I want it to shine I'll just use a paint.

  7. makonate
    Joined: Feb 2014
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    Location: rockport ma

    makonate New Member

    thank you for the advice this has all been really helpful ill post with results in the spring...

  8. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Are you spraying the whole hull or only doing small repairs?
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