Minimum compressor CFM/gal requirements for spraying gel coat?

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

  1. Bonefishr
    Joined: Dec 2012
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    Bonefishr Junior Member

    I apologise. I can't seem to find a definitive answer. Been here for days and learned everything I need to know except what is the smallest compressor I can use to spray gel coat.

    I have five 16' skiffs. I want to regel the hulls on a couple of them. I'm in the Bahamas. I did all the prep on one skiff and hauled it down to the local boat shop to spray the gel. I think they used a couple cans of enamel spray paint. It's like everything in the Bahamas, better to do it yourself. I'm going to take my time and do one at a time. I'm sure I don't need a 100 gal commercial grade compressor, it's overkill and will cost me double what you're paying in the states. I don't think a six gallon could pull it off....maybe I'm wrong. If I could get the same answer twice I'll go with it. Thanks in advance.
     
  2. michael pierzga
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    michael pierzga Senior Member

    Hmm...wouldnt it be easier to paint them ?

    I dont know much about gelcoat. All the small gel repairs I have done with a spray gun were far from perfect and needed subsantial machine polishing. Perhaps there is a pro way to finish with gel.
     
  3. Bonefishr
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    Bonefishr Junior Member

    I did try to brush the gel coat on a different boat once....after I tried rolling the first coat. What a sandiing nightmare, never again! I'm resigned to the fact that I'll have to do plenty of sanding regardess of the application method. Spraying can't be as bad as rolling or brushing. In this day and age all that sanding seems very primitive. You'd think someone would have figured out how to shoot gel without having to sand it. The first guy that does will be a billionaire.
     
  4. rxcomposite
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    rxcomposite Senior Member

    That is because it depends on the type of work you do. It is the pressure and the capacity that is important.

    I have a 1.5 Hp. single head, 20 gallon, 45 psi compressor that I use for repair work. Can barely spray continously a 10 square foot panel even when the gel coat is thinned.

    In the shop, we use a 5 Hp two head, 100 gallon, 90 psi compressor that can spray gel coat on 20-25 footer boat. Have to stop once in a while and we spray only a couple of boats a week. I burned the motor (2 in a day) when I tried to spray continously on a 90 footer cat.

    On the other factory, we spray 60 footer hull. We have 3 banks of 5 hp, 2 heads, 100 gallon, 90 psi compressor. One of the 3 keeps burning out.

    Still on another factory, we are using a monster, 15 hp, 3 phase, 200 gallon, 2 heads, 120 psi branded compressor to spray 30 footer cat hulls almost daily. Never had a problem for 10 years. But we have a gel coat sprayer for the big hulls so the compressor even when used daily, is not pushed to the limit.

    So for your 16 footer, a 5hp, 100 gallon, 90 psi will not give you problem. Anything less, you have to take it easy on the compressor.
     
  5. Bonefishr
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    Bonefishr Junior Member

    Hence my confusion. Google the question and you get all kinds of answers. The next guy will say he did his boat with his nail gun compressor with no problem. That's hard to believe. Look at it this way...I'm just one guy trying to gel the hull on one little boat. After that the only good that compressor will do me when I'm done is to put air in my tires. I don't need a commercial grade compressor that'll blow gel coat all day long seven days a week. These Bahamians use their dive compressors with no storage tank. If it were summertime I know I'd need at least a 50gl and have to move fast. It's winter, the days are cool down here so the gel won't kick right away. I have time to wait for the compressor to catch up....to a point. If the pressure starts drops too fast seconds after I pull the trigger I'm going to have a mess. I'm trying to find a happy medium. I suppose I'll get a 10-20 gl and hope for the best.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Try roller coating the gel coat. You might have to make two passes to even out the streaks. The coating will be thicker compared to the sprayed one but beats the hell out of buying a compressor.

    I think you are gel coating on the outside, right? Why not just rent or borrow an air compressor. Saves you from making a costly investment.
     
  7. michael pierzga
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    michael pierzga Senior Member

    What is the proper gel coat viscosity for a rolled gelcoat application ?
     
  8. rxcomposite
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    rxcomposite Senior Member

    I don't use viscosity meter. I just thin it down so it rolls easy without being too lumpy. when roller coating to a mold, 15 to 18% by volume works for me. Not more than 30% as indicated in the material specs.

    You will not be worried about the roughness when working from a mold. It has to be even otherwise the streaks shows up when viewed through a sunlight. Working on the surface of a part, you have to thin it down more and apply at least two coats and use a wax solution for the last coat. Most of the gel coat will be removed by sanding to get a glossy finish.
     
  9. Bonefishr
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    Bonefishr Junior Member

    Yep....I'm gel coating the outside hull.

    I tried the roller, it slides more than it rolls. Might as well wipe it on with a rag. I brushed the second coat and reduced the kicker hoping the brush marks would level out. It didn't level at all. I sanded most of the gel off that I put down trying to level it. No way will I roll or brush again.

    I'm located on an out island. These people are living in shacks. They used bicycle pumps. I have the best compressor in town, it's a small battery powered one. One of the local boys just came over some air. The only 'real' compressor belongs to a mechanic twenty miles up the road...he's not going to loan it out.

    Again.....just wanna know what the smallest compressor is that will work "good enough".
     
  10. Herman
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    Herman Senior Member

    Do you thin down with Duratec High Gloss additive? Saves a lot of sanding...
     
  11. midnitmike
    Joined: Apr 2012
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    midnitmike Senior Member

    Bonefishr,
    I use a small 13 gal compressor for my gelcoat spraying, and it works but I do spend time waiting for it to catch up. Your compressor demands are going to vary depending on the gun you're using (tip size) and the viscosity of the gelcoat. If I've noticed anything over time it's that the more I cut the gelcoat (looking for that paint like finish) the softer the final product. It seems more porous and less dense then full strength gelcoat, probably due to the excessive amounts of MEK, or acetone needed to get it through the gun.

    Although I've never used the product myself I would suggest you look at using duratech for your project. It should make spraying easier, and give you a better looking top coat then shooting gelcoat alone.

    http://www.fibreglast.com/product/duratec-clear-hi-gloss-gel-coat-additive/

    If it works as advertised you may not have to sand at all...making it well worth the investment.

    MM
     
  12. Bonefishr
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    Bonefishr Junior Member

    I don't know much about the duratec. I'll google it.

    Fiberglass Coatings out of Florida sold me a couple quarts of styrene and told me to thin it with that.

    After spending a couple days reading in forums I'm under the impression to not thin it at all. One guy says use acetone, the other says no way. One guy says use styrene the other says it'll yellow the gel. Either one changes the chemical properties that'll lead to all kinds of other problems. You can imagine I'm ready to pull some hair out.
     
  13. Bonefishr
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    Bonefishr Junior Member

    Hey midnitmike,

    Thanks! Now where getting somewhere. At least you're telling what I want hear.:)

    These boats are work boats....bonefishing guides pushing them through skinny water all day. I want to do it properly but they don't have to be perfect. I have a torn up shoulder, I'd like to avoid sanding at all if I can.
     
  14. midnitmike
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    midnitmike Senior Member

    Bonefishr,
    I've had the same advice over the years, use this don't use that, it seems that everyone has an opinion and they're all right. I've shot with acetone, styrene, and MEK and to honest I'm not impressed with any of them as a reducer. By the time you add enough of any of these products to get the gelcoat through the gun you greatly affect it's properties. Since you intend to paint the exterior of these hulls, and want to reduce the sanding normally associated with such a task I would highly recommend a product designed for just this.

    In Wave Whacker's thread on a jet boat conversion he used this to good effect as his finish coat, once they ironed out the application technic.

    MM
     

  15. Bonefishr
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    Bonefishr Junior Member

    I looked over the Duratec specs. One question wasn't answered....can you add Duratec to gel coat with wax?....Rather than adding styrene wax for below waterline apps. I called Duratec. The sales rep didn't know. Guess I'll mix the two and see if they explode.
     
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