Using light Fg cloth to protect Carbon layer

Discussion in 'Fiberglass and Composite Boat Building' started by idkfa, Apr 14, 2013.

  1. idkfa
    Joined: Sep 2005
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    Location: Windward islands, Caribbean

    idkfa Senior Member

    It is normal to protect a light carbon schedule with a layer of light Fg cloth, what weight is appropriate? I'm considering using a 2oz / 75gsm cloth as a sacrificial sanding layer in a female mould vacuum bag wet layup. The hull will be primed and painted and concerned that when sanding the primer, one may cut the carbon fibres. Or maybe a sacrificial layer of carbon is more advantageous ie. lighter and easier to work with -uni versus woven?

    I was advised against using primer in the mould due to contamination with the mould release wax.

    thanks,
     
  2. dinoa
    Joined: Oct 2007
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    Location: florida

    dinoa Senior Member

    This is often done with 50 gm/m2 on sailplane wings to prevent print through of more course cloth below but you'll cut through it almost immediately when sanding if not careful.

    Dino
     
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  3. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    I have specified glass over carbon before for exactly the reasons you suggest, although usually in a male-molding situation where the final glass layer is the last one laid up. In these situations, 6 oz to 10 oz/sq.yd (200 gsm - 340 gsm) is my typical weight. I figure that roughly half the thickness is lost during sanding. So original glass lay-up is about 0.008" to 0.012" (0.2 to 0.3 mm), and it reduces to 0.004" to 0.006" (0.1 to 0.15 mm) after sanding. And at that, some spots will break through exposing the carbon during sanding.

    In a female molding situation, I would expect that a 2 oz (75 gsm) glass layer may not have a really good surface coming out of the mold--might be a lot of pin holes in it. Plus, on sanding, you really won't have anything left--it'll all get sanded off. It will only be about 0.003" (.08 mm) to begin with, and that is hardly anything at all. Sandpaper will cut through it pretty quickly.

    I'd say go ahead with a sacrificial glass, but use a heavier fabric so that you have a better chance of a good sandable surface when the part is out of the mold, and which will definitely leave a bit of glass on the surface after sanding for your fairing materials. Also, I encourage you to do a few test panels in the mold with different glass and carbon fiber mixes (don't need much carbon, but test different glass layers and thicknesses) and after molding, try sanding them and see which comes out the best. You won't have to go heavier than 10 oz, and likely not heavier than 6 oz.

    Good luck. Report back what you find.

    Eric
     
  4. idkfa
    Joined: Sep 2005
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    Location: Windward islands, Caribbean

    idkfa Senior Member

    I've done a couple of test, and Eric your right about the pin-holes, theres lots of them.

    Sanding though, just enough to roughen the surface for adhesion of primer, and using 100grit sandpaper too, works, does cut through the Fg. I've tried this sanding on the carbon too, and it is only surface scratches, does not apparently ruin the layer -150gsm.

    The peel strength between the Fg and carbon was very surprising low, ripped right off like peel-ply!

    Will omit Fg layer altogether, uses lots of epoxy, heavy, and an additional layer of carbon will more than stand-up to the sanding, and maybe add less weight than the Fg layer.

    Thanks,
     
  5. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    Thanks for your report. Yes, very light glass fabrics act like peel ply, which is another bad thing about using them. And for your further information, the carbon fiber spar builders typically do not use any sacrificial glass in their lay-ups. The final finish is direct sanding of the carbon fiber surface, then filling, fairing and painting. So you are in good company.

    Eric
     

  6. rxcomposite
    Joined: Jan 2005
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    Location: Philippines

    rxcomposite Senior Member

    We have used fiberglass veil cloth prepreg in vacuum bagged oven cured process to get a good surface finish. Even a thin coat of sanding gel coat (gel coat loaded with micro ballons) will work. Just remember to be carefull when sanding. That is, you have to sand along the direction of the fibers. If the fibers are showing, you are sanding too much.
     
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