Minimum compressor CFM/gal requirements for spraying gel coat?

Discussion in 'Fiberglass and Composite Boat Building' started by Bonefishr, Dec 19, 2012.

  1. midnitmike
    Joined: Apr 2012
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    midnitmike Senior Member

    Bonefishr,
    There have been instances where I needed non waxed gelcoat, and had none on hand...what to do? Since wax tends to rise to the surface of gelcoat if it's been sitting a while what I did was "Carefully" pour and spoon off this free floating wax. granted I couldn't get it all, but it's amazing how much you can remove. For those final bits I couldn't get with a spoon, I wicked it up with a paper towel. Of course I saved the liquid wax for use later on, but I can report reasonable success with this method.

    MM
     
  2. waikikin
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    waikikin Senior Member

    http://www.duratec1.com/dp07.html

    No need for wax in styrene when you use Duratec, here's the link, I've used the high gloss additive a few times although never to the 1:1 ratio they quote. It makes the gelcoat like paint but it's less opaque than plain gelcoat so I just put enough in to be right for me. Their other plug finishing systems are fantastic. Jeff.
     
  3. Bonefishr
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    Bonefishr Junior Member

    Jeff...I'd assume the Duratec thins the gel coat. Can I still get by with a 2.3-2.5ish spray tip or will I need something smaller?
     
  4. waikikin
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    waikikin Senior Member

    Good question;) best thing to do is a couple of small experiments with the gear you have, which means "sneaking up" on the process, a bloke I used to work for used to use the same chinese gun with a 2.5 tip for everything, contact cement, gelcoat, enamel, high build primer, all his work was terrific & he adjusted the gun & thinners to suit. If it's a gravity gun you have I'd start with samples of 100ml of gelcoat, add to them 25,50,75 & 100ml of duratec, add the cat to the +100 sample & test shoot it, then the 75 then the 50 then the 25 & adjust your fluid & pattern to each, maybe do your tests across a transom & come back to it & assess the surface of each area for gloss, sandability peel & remember to record the ease of spraying the material, larger batches can cook up in your gun if it takes to long, you might need larger test batches to assess your process. I only spray stuff on an occassional basis, spraying is a whole trade on its own & those that do it on a daily basis get really good at it naturally but if you take some time up front to test & be prepared to stop if its shooting like $hit(cos thats more work to fix), also have acetone & containers ready to clean your gun if it starts a cook up. Regards from Jeff.
     
  5. rxcomposite
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    rxcomposite Senior Member

    Why not just prep and paint the boat? Gel coat is great for molded in product but it does not polish well if sprayed on the outside. Paint does a better job and you don't need a large compressor.
     
  6. tunnels

    tunnels Previous Member

    Compressor for spray painting 15 cfm is about the minimum size . Pressure you need about 50psi its mainly volumn you need !!
    There nothing wrong with a good quality 2 pot polyurethane !!most 2 pots urethans are as hard as Gel coat any way . thick gel will get hair line cracks if it got to much thickness ! personally i would 2 pot !!:D
     
  7. ondarvr
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    ondarvr Senior Member

    Every type of thinner used will have effects on the gel coat and they will vary depending on how much is added. You may or may not see the possible issues short term, but long term there can be problems. It’s a trade off, easier application, sanding and buffing for a possible shorter lifespan.

    Acetone inhibits the cure, if you add small amounts chances are most of it will evaporate while being sprayed, if it doesn’t evaporate and is trapped in the film, the gel coat may never cure completely. The other issue is it can change the color or cause the pigments to separate. The problem is if someone says it’s OK to add 3% it rapidly turns into 30% on the shop floor, the repair may look OK short term, but UV, water and crack resistance can be greatly reduced.

    Styrene doesn’t inhibit the cure in the same way, but it does reduce the physical properties, plus can make it a little more brittle, so cracking can be more likely. It also reduces the UV resistance and loves to turn yellow.

    MEK, don’t know exactly, most don’t use it. It does similar things as acetone though, but doesn’t evaporate as quickly, so more will be left in the gel coat as it cures.

    Duratec….. Color change, less UV resistance, less water resistant, not sure on the physical properties.
    It can yield a glossier finish because the pigment loading has been reduced, pigments don’t shine well even when buffed, resin does. The finish may look deeper and richer, the reduced pigment loading will allow you to see deeper in the finish. This may also result in poor hide and a blotchy finish if you don’t spray it thick enough.

    The best products to use are ones that are designed for this purpose, they rely on unthixed resin similar to the base resin used to make the gel coat to reduce the viscosity, so the physical properties aren’t reduced, the color may change a little due to reducing the pigment % though.

    With any of these add as little as possible to get the desired results. Most people add far too much.

    You will need to use a pressure pot to apply the gel coat, a gravity or siphon feed gun won’t deliver enough product when doing more than a small area. Pressure pots use less air volume too, so you can get by with a smaller compressor.

    Another option is to rent an electric paint sprayer, the type you would use for a house, you just need to be very careful to not let it start to gel before you finish, or you will have just purchased a sprayer.
     
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  8. OFFSHORE GINGER
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    OFFSHORE GINGER Junior Member

    www.spraygunworld.com and when you spray gel the bigger the tip the better 2.5 , 2.8 0r 3.0 and the option of using a electric spray gun is not very good or sound advise considering some solvents ( acetone ) will , and can melt the gun down .
     
  9. waikikin
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    waikikin Senior Member

    About 20 years ago I tried shooting some gelcoat with one of these p://www.krebsswitzerland.com/en/paint-spray-guns/krebs-25-powerful-semi-professional-paint-spray-gun.html , it was marketed by Oldfields in Aus, went great for about a minute or so until the heat generated by the piston/cylinder arrangment cooked & set the gelcoat, the gel in the pot was still fine & I finished the smallish job with a brush(contact molded transom extension) but the electric gun was nfg. Jeff.
     
  10. ondarvr
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    ondarvr Senior Member

    I have an airless paint sprayer (house type) that is used just for this purpose, never had a problem with acetone in any unit I have ever used (many). I don't always recommend it because you need to be careful to not set it up, other than that no problems. I have have only used commercial models, a cheap homeowner unit may not cut it.
     
  11. ondarvr
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    ondarvr Senior Member

    If you let the pump cycle it will get hot, or if you stop spraying for too long the pump heat may cause it to gel sooner. If you keep sparying the pump doesn't have enough time to heat the gel coat. Again, you need to know how to do it, no different than the first time someone uses a pressure pot, it frequently ends with many 4 letter words and a trip back to the store to buy new hoses and such.

    I did set one up one time, I needed to spray many 5 gallon pails of gel coat at one time on a remote job site, on the second or third pail a breaker tripped...it was over before I was back to pull the trigger. I was doing this job by myself, didn't have the normal backup guy to handle any details like this.
     
  12. harmonpa
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    harmonpa New Member

    Might be helpful here is a guide on spraying gelcoat on mentioned you atleast need a pressure pot and spray gun with a larger tip or sometimes a gravity gun if they have 2.5 mm tips (very large) though this will be useful only for touch up work
     
  13. wet feet
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    wet feet Senior Member

    One thing nobody has mentioned on this rather old thread is that if you spray gelcoat,everything and I mean everything in the workshop will become sticky.At a minimum you need to set up a plastic tent around the job with good extraction.If you are determined to use a spray system it would be sensible to talk to a technical sales representative of the company supplying the material.
     
  14. gonzo
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    gonzo Senior Member

    You don't need wax with Duratec Clear Additive. Mix it 50-50 with gelcoat and it sprays like enamel. The gallons of the compressor's tank are irrelevant. What you need is the sufficient air flow (CFM) and pressure for the spray gun you are using. With a gravity feed you'll need about 6CFM @ 40 PSI.
     

  15. gonzo
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    gonzo Senior Member

    You don't need wax with Duratec Clear Additive. Mix it 50-50 with gelcoat and it sprays like enamel. The gallons of the compressor's tank are irrelevant. What you need is the sufficient air flow (CFM) and pressure for the spray gun you are using. With a gravity feed you'll need about 6CFM @ 40 PSI.
     
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