Guidance on Sanding Vacuum Bagged Hard Top

Discussion in 'Fiberglass and Composite Boat Building' started by fly186, Jan 3, 2017.

  1. fly186
    Joined: Nov 2016
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    Location: Florida

    fly186 Junior Member

    Rather than start another thread, and since I'm still working on the same top, I figured I would post this question here...

    I need to smooth out my top in preparation for painting so the final layer of 1708 I added for strength needs to be sanded... but how much?

    I will be using some fairing compound (Awlfair) as needed but I would like some guidance on how much to sand the 1708. Should I lightly hit it with 80 grit or a torture board all over or use the DA to just take off the binder threads sticking up? The top will be getting 2 coats of Algrip 545 primer also and that will be sanded too.
    I guess the question is should I try to grind the glass close to smooth or just use the 545 and Awlfair to fill in the weave?
    I don't want to weaken the 1708 by grinding into it too much but I also don't want to add a lot of weight with fairing compounds.

    Here's a picture of the 1708...

    20170209_110546_resized.jpg

    Thanks
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Just kiss the binder threads with a DA, as you don't want to nick the biax fibers. Once the DA has done its job, use a flap wheel (lightly) with a relatively fine grit, say 100, so you can get down into the weave, without too much fiber shredding. Now you'll be ready for fairing compound and the board of pain.
     
  3. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    Thanks PAR but I'm not sure I'm clear on this. You said to hit it with a DA to get the binder fibers and then switch to a flap wheel to "get down into the weave". Maybe the advice is about using sandpaper that isn't too rough and would rip up those fibers instead of sanding them smooth. Not sure but I think I want to just "level" things out with the DA and knock down the tops of the weave. After that I plan to roll on a very thin coat of Alwgrip 545 and sand lightly before spreading on a very thin coat of Awlfair LW to fill in the weave. Then more sanding of the fairing compound followed by a few more coats of 545 with the last being sanded to 320 at which time this damned thing will be ready for the spray booth!
    Does this make sense?

    I've also been told not to use the Awlfair LW and use epoxy with microballons instead since it's easier to sand.
    Anyone have pro's and con's related to these two options?

    Thanks!
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The DA will knock down the polyester threads, holding the biax together, while the flap wheel is very effective at getting down in the weave without much damage. If you just kiss the tops, the bond will be weaker. Your schedule of more goo, a light sanding then fairing compound, followed by a sealing coat, will do fine. I don't have much experence with Awlfair LW, typically making my own instead.
     
  5. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    Well, after 3 days of grinding, sanding, priming, fairing, sanding, fairing, and more sanding... I'm looking to hire a pro to finish fairing the top. I consider myself a skilled amateur in a lot of areas and a quick study but I know when I'm beat. I could spend another 3 days on this top and it might be close but still not right. I work 50 hrs/week and have spent the last 6 weekends on this thing. It needs to get done so I can put the boat back together and go fishing!

    BTW... IMHO, Awlfair LW is a pain to work with mainly because it takes 6 hrs to be ready for sanding and it's also hard stuff.

    topfaired.jpg
     
  6. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    There are some epoxy compatible sanding primers that are much easier to use, they cure quick and are easy to sand.
     
  7. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    Yes, a friend just told me to consider using Interlux Watertite Epoxy Filler instead. I've used the 3M filler but it's a VE resin and while it would be fine I just feel better sticking with epoxy-based fillers. I also have some Totalboat TotalFair compound so may try that also.
    Fortunately the bottom of the top is very smooth and will require very little fairing since it was against the mold surface when this thing was vacuum bagged.
    When fairing/sanding is done it will get 3 coats of 545 and then be painted with Awlcraft 2000... assuming it ever gets faired!
     
  8. ondarvr
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    ondarvr Senior Member

    check your PMs
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    System Three "QuickFair" is also another easy to sand filler.

    Fairing is mostly about the process. What most novices do is sand too much or not effectively. This is technique based, not so much product, though having something easy to sand is helpful.

    One trick some use (I don't very often) is to apply the filler with a notched trowel. A longboard is taken to this to level the area to the high spots, knowing the lows are filled. With this done another application is applied to fill the notches and fine tune the fairing job. I find this wasteful and heavy, but it does work on really rough surfaces.
     

  10. fly186
    Joined: Nov 2016
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    fly186 Junior Member

    Very true and I'm not sure I have the time to learn the technique on this job. Unfortunately the pro I tried to hire is tied up for a few weeks.
    I'm going to try one more application of fairing compound but this time with a long (36") aluminum straight edge to spread the material evenly and then make a longer fairing board of about the length for sanding.
     
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